Follow-up to #17805
State: closed

This replaces uses of pandas_datetime_to_datetimestruct and pandas_datetimestruct_to_datetime with dt64_to_dtstruct and dtstruct_to_dt64 respectively, but leaves some un-replaced to avoid creating conflicts with other open PRs.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry


Labels:
name: Datetime
description: Datetime data dtype
name: Internals
description: Related to non-user accessible pandas implementation

Comments:
looks fine. ping when green.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=h1) Report
> Merging [#18023](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a355ed2a8c32a29c1dc93466f9b65870cac272fa?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18023/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18023      +/-   ##
==========================================
- Coverage   91.26%   91.24%   -0.02%     
==========================================
  Files         163      163              
  Lines       50099    50099              
==========================================
- Hits        45724    45715       -9     
- Misses       4375     4384       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.05% <ø> (ø)` | :arrow_up: |
| #single | `40.24% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=footer). Last update [a355ed2...04dd69d](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=h1) Report
> Merging [#18023](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a355ed2a8c32a29c1dc93466f9b65870cac272fa?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18023/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18023      +/-   ##
==========================================
- Coverage   91.26%   91.24%   -0.02%     
==========================================
  Files         163      163              
  Lines       50099    50099              
==========================================
- Hits        45724    45715       -9     
- Misses       4375     4384       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.05% <ø> (ø)` | :arrow_up: |
| #single | `40.24% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=footer). Last update [a355ed2...04dd69d](https://codecov.io/gh/pandas-dev/pandas/pull/18023?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

ping
thanks

i assume u have a note to change the remaining on tslib at some point