transition period_helper to use pandas_datetimestruct
In the last round we transitioned period.pyx to use pandas_datetimestruct instead of date_info.  This does the same thing for period_helper.
lgtm. ping on green. actually, any doc-strings to update? > actually, any doc-strings to update?

The affected docstrings are already updated. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19918?src=pr&el=h1) Report
> Merging [#19918](https://codecov.io/gh/pandas-dev/pandas/pull/19918?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9958ce68a19477721d2ba53bde2b17bb52ebebaa?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19918/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19918?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19918      +/-   ##
==========================================
+ Coverage   91.66%   91.68%   +0.02%     
==========================================
  Files         150      150              
  Lines       49026    49026              
==========================================
+ Hits        44939    44951      +12     
+ Misses       4087     4075      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.06% <ø> (+0.02%)` | :arrow_up: |
| #single | `41.83% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19918?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19918/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19918?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/19918?src=pr&el=footer). Last update [9958ce6...e16dd3e](https://codecov.io/gh/pandas-dev/pandas/pull/19918?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 you need to rebase on master Ping thanks!