canonicalize data dir in config ID hash

fixes #5871 

The second commit is optional but improves readability.

Comments:
_The documentation is not available anymore as the PR was closed or merged._

<details>
<summary>Show benchmarks</summary>

PyArrow==8.0.0

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.009137 / 0.011353 (-0.002216) | 0.006119 / 0.011008 (-0.004889) | 0.136530 / 0.038508 (0.098022) | 0.038434 / 0.023109 (0.015325) | 0.427900 / 0.275898 (0.152002) | 0.449757 / 0.323480 (0.126277) | 0.007673 / 0.007986 (-0.000313) | 0.007147 / 0.004328 (0.002818) | 0.108029 / 0.004250 (0.103778) | 0.055072 / 0.037052 (0.018020) | 0.439245 / 0.258489 (0.180756) | 0.477285 / 0.293841 (0.183444) | 0.044838 / 0.128546 (-0.083708) | 0.020814 / 0.075646 (-0.054832) | 0.436098 / 0.419271 (0.016826) | 0.067459 / 0.043533 (0.023926) | 0.427470 / 0.255139 (0.172331) | 0.443260 / 0.283200 (0.160060) | 0.125466 / 0.141683 (-0.016216) | 1.996756 / 1.452155 (0.544601) | 2.100679 / 1.492716 (0.607962) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.278407 / 0.018006 (0.260401) | 0.625855 / 0.000490 (0.625365) | 0.005544 / 0.000200 (0.005344) | 0.000107 / 0.000054 (0.000053) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.033495 / 0.037411 (-0.003916) | 0.134718 / 0.014526 (0.120192) | 0.150151 / 0.176557 (-0.026406) | 0.221385 / 0.737135 (-0.515751) | 0.150932 / 0.296338 (-0.145406) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.668845 / 0.215209 (0.453636) | 6.678436 / 2.077655 (4.600781) | 2.714074 / 1.504120 (1.209954) | 2.275784 / 1.541195 (0.734589) | 2.332852 / 1.468490 (0.864361) | 1.014877 / 4.584777 (-3.569900) | 6.086455 / 3.745712 (2.340743) | 2.990029 / 5.269862 (-2.279832) | 1.862236 / 4.565676 (-2.703441) | 0.122179 / 0.424275 (-0.302096) | 0.015706 / 0.007607 (0.008099) | 0.873473 / 0.226044 (0.647429) | 8.580109 / 2.268929 (6.311180) | 3.458360 / 55.444624 (-51.986264) | 2.738801 / 6.876477 (-4.137676) | 2.918428 / 2.142072 (0.776356) | 1.224910 / 4.805227 (-3.580317) | 0.243006 / 6.500664 (-6.257658) | 0.087121 / 0.075469 (0.011652) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.757802 / 1.841788 (-0.083986) | 19.447999 / 8.074308 (11.373691) | 24.518157 / 10.191392 (14.326765) | 0.245013 / 0.680424 (-0.435411) | 0.032290 / 0.534201 (-0.501911) | 0.542043 / 0.579283 (-0.037240) | 0.708154 / 0.434364 (0.273790) | 0.660584 / 0.540337 (0.120247) | 0.794868 / 1.386936 (-0.592068) |

</details>
PyArrow==latest

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.009496 / 0.011353 (-0.001857) | 0.005842 / 0.011008 (-0.005166) | 0.112813 / 0.038508 (0.074305) | 0.039120 / 0.023109 (0.016011) | 0.489717 / 0.275898 (0.213819) | 0.532586 / 0.323480 (0.209107) | 0.007681 / 0.007986 (-0.000304) | 0.005337 / 0.004328 (0.001009) | 0.107244 / 0.004250 (0.102994) | 0.056847 / 0.037052 (0.019794) | 0.499447 / 0.258489 (0.240958) | 0.548995 / 0.293841 (0.255154) | 0.058047 / 0.128546 (-0.070499) | 0.015468 / 0.075646 (-0.060179) | 0.124600 / 0.419271 (-0.294671) | 0.060940 / 0.043533 (0.017407) | 0.488370 / 0.255139 (0.233231) | 0.518540 / 0.283200 (0.235341) | 0.124147 / 0.141683 (-0.017536) | 1.902922 / 1.452155 (0.450767) | 2.033519 / 1.492716 (0.540803) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.319527 / 0.018006 (0.301521) | 0.629641 / 0.000490 (0.629152) | 0.000721 / 0.000200 (0.000521) | 0.000101 / 0.000054 (0.000046) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.033150 / 0.037411 (-0.004262) | 0.134250 / 0.014526 (0.119724) | 0.161273 / 0.176557 (-0.015283) | 0.211471 / 0.737135 (-0.525664) | 0.155326 / 0.296338 (-0.141012) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.705244 / 0.215209 (0.490035) | 7.043040 / 2.077655 (4.965386) | 3.308948 / 1.504120 (1.804828) | 2.885050 / 1.541195 (1.343855) | 2.810260 / 1.468490 (1.341770) | 1.027095 / 4.584777 (-3.557682) | 6.111398 / 3.745712 (2.365686) | 5.385545 / 5.269862 (0.115684) | 2.521668 / 4.565676 (-2.044009) | 0.122419 / 0.424275 (-0.301856) | 0.016376 / 0.007607 (0.008768) | 0.830856 / 0.226044 (0.604811) | 8.952199 / 2.268929 (6.683271) | 4.207875 / 55.444624 (-51.236749) | 3.346624 / 6.876477 (-3.529853) | 3.395316 / 2.142072 (1.253244) | 1.351816 / 4.805227 (-3.453411) | 0.303056 / 6.500664 (-6.197608) | 0.098713 / 0.075469 (0.023244) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.841903 / 1.841788 (0.000116) | 20.472125 / 8.074308 (12.397817) | 23.433200 / 10.191392 (13.241808) | 0.242599 / 0.680424 (-0.437825) | 0.030701 / 0.534201 (-0.503500) | 0.541614 / 0.579283 (-0.037669) | 0.657827 / 0.434364 (0.223463) | 0.652448 / 0.540337 (0.112111) | 0.773743 / 1.386936 (-0.613193) |

</details>
</details>

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