Move timedelta-specific functions to tslibs.timedeltas

[jbrockmendel]

Once #17793 is merged then the remaining timedeltas functions can be moved: convert_to_timedelta64 and array_to_timedelta64.

Most of Timedelta and _Timedelta can also be moved over in a follow up.

[codecov]

Codecov Report

Merging #17827 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17827      +/-   ##
==========================================
+ Coverage   91.23%   91.24%   +<.01%     
==========================================
  Files         163      163              
  Lines       50113    50113              
==========================================
+ Hits        45723    45726       +3     
+ Misses       4390     4387       -3

Flag	Coverage Δ
#multiple	89.05% <ø> (+0.02%)	⬆️
#single	40.31% <ø> (-0.06%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.75% <0%> (-0.1%)	⬇️
pandas/plotting/_converter.py	65.2% <0%> (+1.81%)	⬆️

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[jbrockmendel]

This is cut/paste, can be cut down if the diff is too large for easy review.

[jreback]

looks ok, needs a rebase

[jreback]

thanks!