Support for multiple configs in packaged modules via metadata yaml info

will solve https://github.com/huggingface/datasets/issues/5209 and https://github.com/huggingface/datasets/issues/5151 and many other...

Config parameters for packaged builders are parsed from `“builder_config”` field in README.md file (separate firs-level field, not part of “dataset_info”), example:
```yaml
---
dataset_info:
...
configs:
  - config_name: v1
    data_dir: v1
    drop_labels: true
  - config_name: v2
    data_dir: v2
    drop_labels: false
```

I tried to align packaged builders with custom configs parsed from metadata with scripts dataset builder as much as possible. Their builders are created dynamically (see `configure_builder_class()` in load.py`) and have `BUILDER_CONFIGS` attribute filled with `BuilderConfig` objects in the same way as for datasets with script.

## load_dataset
1. If there is single config in meta and it doesn’t have a name, the name becomes “default” (as we do for “dataset_info”), [example](https://huggingface.co/datasets/polinaeterna/audiofolder_one_default_config_in_metadata/blob/main/README.md):
```python
load_dataset("ds") == load_dataset("ds", "default")   # load with the params provided in metadata

load_dataset("ds", "random name")  # ValueError: BuilderConfig 'random_name' not found. Available: ['default']
```
2. If there is single config in metadata with `config_name` provided, it becomes a default one (loaded when no `config_name` is specified, [example](https://huggingface.co/datasets/polinaeterna/audiofolder_one_nondefault_config_in_metadata)
```python

load_dataset("ds") == load_dataset("ds", "custom")   # load with the params provided in meta

load_dataset("ds", "random name")  # ValueError: BuilderConfig 'random_name' not found. Available: ['custom']
```
3. If there are several configs in metadata with names [example](https://huggingface.co/datasets/polinaeterna/audiofolder_two_configs_in_metadata/blob/main/README.md)
```python
load_dataset("ds", "v1")  # load with "v1" params

load_dataset("ds", "v2")  # load with "v2" params

load_dataset("ds")  # ValueError: BuilderConfig 'default' not found. Available: ['v1', 'v2']

```
Thanks to @lhoestq and [this change](https://github.com/polinaeterna/datasets/pull/1), it's possible to add `"default"` field in yaml and set it to True, to make the config a default one (loaded when no config is specified):
```yaml
configs:
  - config_name: v1
    drop_labels: true
    default: true
  - config_name: v2
...
```
then `load_dataset("ds") == load_dataset("ds", "v1")`.

## dataset_name and cache
I decided that it’s reasonable to add a `dataset_name` attribute to `DatasetBuilder` class which would be equal to `name` for scripts dataset but reflect a real dataset name for packaged builders (last part of path/name from hub). This is mostly to reorganize cache structure (I believe we can do this in the major release?) because otherwise, with custom configs for packaged builders which were all stored in the same directory, i it was becoming a mess. And in general it makes much more sense like this, from datasets server perspective too, though it’s a breaking change 

So the cache dir has the following structure: `"{namespace__}<dataset_name>/<config_name>/<version>/<hash>/"` and arrow/parquet filenames are also `"<dataset_name>-<split>.arrow"`. 
For example `polinaeterna___audiofolder_two_configs_in_metadata/v1-5532fac9443ea252/0.0.0/6cbdd16f8688354c63b4e2a36e1585d05de285023ee6443ffd71c4182055c0fc/` for `polinaeterna/audiofolder_two_configs_in_metadata` Hub dataset, train arrow file is `audiofolder_two_configs_in_metadata-train.arrow`.

For script datasets it remains unchanged.

## push_to_hub
To support custom configs with `push_to_hub`, the data is put under directory named either as `<config_name>` if `config_name` is **not** "default" or "data" if `config_name` is omitted or "default" (for backward compatibility). `"builder_config"` field is added to README.md, with `config_name` (optional) and `data_files` fields. for `"data_files"`, `"pattern"` parameter is introduced, to resolve data files correctly, see https://github.com/polinaeterna/datasets/pull/1.
 - `ds.push_to_hub("ds")` --> one config ("default"), put under "data" directory, [example](https://huggingface.co/datasets/polinaeterna/push_to_hub_single_config/blob/main/README.md)
```yaml
dataset_info:
...
configs:
  data_files:
  - split: train
    pattern: data/train-*
...
``` 
- `ds.push_to_hub("ds", "custom")` --> put under "custom" directory, [example](https://huggingface.co/datasets/polinaeterna/push_to_hub_singe_nondefault_config/blob/main/README.md)

```yaml
configs:
  config_name: custom
  data_files:
    - split: train
      path: custom/train-*
...
``` 

 - for many configs, [example](https://huggingface.co/datasets/polinaeterna/push_to_hub_many_configs/blob/main/README.md):
```yaml
configs:
  - config_name: v1
    data_files:
      - split: train
        path: v1/train-*
...
  - config_name: v2
    data_files:
      - split: train
        path: v2/train-*
...
```
Thanks to @lhoestq and https://github.com/polinaeterna/datasets/pull/1, when pushing to datasets created **before** this change, README.md is updated accordingly (config for old data is added along with the one that is being pushed).  

`"dataset_info"` yaml field is updated accordingly (new configs are added).
This shouldn't break anything!

TODO in separate PRs:
- [x] docs
- [ ] probably update test cli util (make --save_info not rewrite `builder_config` in readme)