Continue porting period_helper; fix leftover asfreq bug
In #19650 we fixed a bug with asfreq but missed a weekly-frequency case.  This fixes and tests that.  I'll see if I can cook up a more thorough set of tests to make sure nothing else slips through the cracks.

Other than that, the main thing this does is substitute in `ORD_OFFSET = WEEK_OFFSET * 7 + 4` and `BDAY_OFFSET = 5 * WEEK_OFFSET + 4` and then cleans up some algebra (which is now OK because we are using non-cdivision floordiv and mod).  As a result we get rid of a bunch of calls that add ORD_OFFSET just to subtract it a few lines later.

Gets rid of CamelCase in tslibs.period, moves towards using pandas_datetimestruct directly and getting rid of the redundant `date_info`.

Catches a RuntimeWarning in the tests; closes #19767.

> easiest prob to remove and do that next pass

sure # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19834?src=pr&el=h1) Report
> Merging [#19834](https://codecov.io/gh/pandas-dev/pandas/pull/19834?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a6183a2d9c8d1c5a83940e891475ac23682be75a?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19834/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19834?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19834   +/-   ##
=======================================
  Coverage   91.58%   91.58%           
=======================================
  Files         150      150           
  Lines       48890    48890           
=======================================
  Hits        44775    44775           
  Misses       4115     4115
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.95% <ø> (ø)` | :arrow_up: |
| #single | `41.78% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19834?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/19834?src=pr&el=footer). Last update [a6183a2...3123c0e](https://codecov.io/gh/pandas-dev/pandas/pull/19834?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > I mean the module name itself.

That makes much more sense.  Mind waiting for the next pass since appveyor has a 12hour+ turnaround? > That makes much more sense. Mind waiting for the next pass since appveyor has a 12hour+ turnaround?

ok