Move timedelta-specific functions to tslibs.timedeltas
Once #17793 is merged then the remaining timedeltas functions can be moved: convert_to_timedelta64 and array_to_timedelta64.

Most of Timedelta and _Timedelta can also be moved over in a follow up.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17827?src=pr&el=h1) Report
> Merging [#17827](https://codecov.io/gh/pandas-dev/pandas/pull/17827?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/77b4bb30ed5afc95f2c264c6b25e96ea8a9a5aee?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17827/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17827?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17827      +/-   ##
==========================================
+ Coverage   91.23%   91.24%   +<.01%     
==========================================
  Files         163      163              
  Lines       50113    50113              
==========================================
+ Hits        45723    45726       +3     
+ Misses       4390     4387       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.05% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.31% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17827?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17827?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17827?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17827?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.2% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17827?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17827?src=pr&el=footer). Last update [77b4bb3...fb907b6](https://codecov.io/gh/pandas-dev/pandas/pull/17827?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 This is cut/paste, can be cut down if the diff is too large for easy review. looks ok, needs a rebase thanks!