Separate out internals.concat, internals.managers

[jbrockmendel]

Follow-up to #22014.

Moved get_blockno_placements to libinternals since a) its a natural fit anyway and b) it is needed in both concat and managers and I didn't want to runtime-import it.

The only non-cut/paste edit is a change of isinstance(self.block, ExtensionBlock) to self.block.is_extension

__init__ namespace is big in part because io.packers uses getattr(internals, name) and is just really tightly intertwined with internals.

[codecov]

Codecov Report

Merging #22028 into master will increase coverage by <.01%.
The diff coverage is 96.88%.

@@            Coverage Diff             @@
##           master   #22028      +/-   ##
==========================================
+ Coverage   91.99%      92%   +<.01%     
==========================================
  Files         168      170       +2     
  Lines       50597    50601       +4     
==========================================
+ Hits        46549    46553       +4     
  Misses       4048     4048

Flag	Coverage Δ
#multiple	90.4% <96.88%> (ø)	⬆️
#single	42.2% <68.1%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/internals/__init__.py	100% <100%> (+3.11%)	⬆️
pandas/core/internals/managers.py	96.52% <96.52%> (ø)
pandas/core/internals/concat.py	98.37% <98.37%> (ø)

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[jreback]

yep, thanks @jbrockmendel