Simplify argument passing in period_helper

[jbrockmendel]

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]

Codecov Report

Merging #19550 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            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%)	⬇️
#single	41.73% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/util/testing.py	83.64% <0%> (-0.21%)	⬇️

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[jbrockmendel]

After this and #19540 we’ll be ready to rip out date_info and just use pandas_datetimestruct directly.

[jreback]

rebase

[jreback]

thanks. ideally we would blow out all of this c-code and cythonize it (I think you are headed this way)