De-privatize tslibs functions

[jbrockmendel]

Related: an item on the tslibs TODO list, #21698.

[codecov]

Codecov Report

Merging #21714 into master will decrease coverage by <.01%.
The diff coverage is 69.23%.

@@            Coverage Diff             @@
##           master   #21714      +/-   ##
==========================================
- Coverage    91.9%    91.9%   -0.01%     
==========================================
  Files         158      158              
  Lines       49690    49688       -2     
==========================================
- Hits        45670    45668       -2     
  Misses       4020     4020

Flag	Coverage Δ
#multiple	90.28% <69.23%> (-0.01%)	⬇️
#single	41.95% <0%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/accessors.py	90.09% <ø> (-0.1%)	⬇️
pandas/core/indexes/datetimelike.py	96.72% <100%> (ø)	⬆️
pandas/core/indexes/period.py	92.57% <62.5%> (-0.02%)	⬇️
pandas/tseries/frequencies.py	95.9% <75%> (ø)	⬆️

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[gfyoung]

cc @jreback

[jreback]

thanks @jbrockmendel