move src/datetime.pxd funcs to np_datetime and fix misleading names
The name `_pydatetime_to_dts` is misleading, since `dts` is the variable names used for `pandas_datetimestruct` objects, while the func actually returns an int64.  This renames it to `_pydatetime_to_dt64`.

The analogous function for `datetime.date` has the non-misleading name `_date_to_datetime64`.  For consistency this is renamed to `_pydate_to_dt64`.

Both are moved to `np_datetime`, and have their inputs typed as `datetime` and `date` instead of `object`.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18045?src=pr&el=h1) Report
> Merging [#18045](https://codecov.io/gh/pandas-dev/pandas/pull/18045?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c65a0f554b0381ac3f50762050a56bcff188ffa1?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/18045/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18045?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18045      +/-   ##
==========================================
- Coverage   91.24%   91.23%   -0.02%     
==========================================
  Files         163      163              
  Lines       50114    50114              
==========================================
- Hits        45729    45720       -9     
- Misses       4385     4394       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <ø> (ø)` | :arrow_up: |
| #single | `40.24% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18045?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18045?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18045?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/18045?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/18045?src=pr&el=footer). Last update [c65a0f5...075574d](https://codecov.io/gh/pandas-dev/pandas/pull/18045?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/18045?src=pr&el=h1) Report
> Merging [#18045](https://codecov.io/gh/pandas-dev/pandas/pull/18045?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c65a0f554b0381ac3f50762050a56bcff188ffa1?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/18045/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18045?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18045      +/-   ##
==========================================
- Coverage   91.24%   91.23%   -0.02%     
==========================================
  Files         163      163              
  Lines       50114    50114              
==========================================
- Hits        45729    45720       -9     
- Misses       4385     4394       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <ø> (ø)` | :arrow_up: |
| #single | `40.24% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18045?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18045?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18045?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/18045?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/18045?src=pr&el=footer). Last update [c65a0f5...ccf793e](https://codecov.io/gh/pandas-dev/pandas/pull/18045?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 ```
taskset 3 asv continuous -E virtualenv -f 1.1 master HEAD -b timeseries
[...]
       before           after         ratio
     [4578a039]       [075574d7]
+           1.61s            1.95s     1.22  timeseries.AsOfDataFrame.time_asof_nan
+        124±20ms            145ms     1.17  timeseries.DatetimeIndex.time_dti_tz_factorize
-      21.6±0.2μs       19.3±0.1μs     0.89  timeseries.Offsets.time_custom_bday_incr
-      27.1±0.2μs       23.9±0.1μs     0.88  timeseries.Offsets.time_custom_bday_cal_incr
-           1.89s            1.66s     0.88  timeseries.AsOfDataFrame.time_asof
-       149±0.2ms        130±0.1ms     0.88  timeseries.ToDatetime.time_iso8601_tz_spaceformat
-      17.6±0.1μs       14.7±0.1μs     0.83  timeseries.Offsets.time_custom_bday_apply
-        14.4±1ms      11.8±0.01ms     0.82  timeseries.DatetimeIndex.time_infer_freq_none
-        21.0±2ms         11.8±1ms     0.56  timeseries.TimeSeries.time_add_irregular
```

Any idea why `time_asof_nan` and `time_dti_tz_factorize` might slow down?  There should be small bumps from specifying types for pydate_to_dt64 and pydatetime_to_dt64, and from cimporting tz_convert_single in offsets.  The only drag I can think of is maybe having funcs in a .pyx instead of .pxd affects how they are inlined? I benchmarked this and it showed very little changes (as expected). something is still going on with your asv. you should *always* use affinity. Thanks for double-checking.