Simplify argument passing in period_helper
Orthogonal to #19540, though will need some small rebasing if/when that goes in.

Instead of passing `relation` argument all over the place, pass it once slightly earlier and make it part of `asfreq_info`.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19550?src=pr&el=h1) Report
> Merging [#19550](https://codecov.io/gh/pandas-dev/pandas/pull/19550?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/36f905285c0089228985cffc9f9f6c7d28789128?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19550/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19550?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19550      +/-   ##
==========================================
- Coverage   91.62%   91.61%   -0.01%     
==========================================
  Files         150      150              
  Lines       48780    48780              
==========================================
- Hits        44693    44691       -2     
- Misses       4087     4089       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.98% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.73% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19550?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19550/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.64% <0%> (-0.21%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19550?src=pr&el=footer). Last update [36f9052...64fc567](https://codecov.io/gh/pandas-dev/pandas/pull/19550?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 After this and #19540 we’ll be ready to rip out date_info and just use pandas_datetimestruct directly. rebase thanks. ideally we would blow out *all* of this c-code and cythonize it (I think you are headed this way)