Separate io helpers from _libs.lib

[jbrockmendel]

Avoids overlap with #19360 where possible.

Moved functions are cut/paste verbatim.

I'm open to a better name than io_helpers, just wanted to avoid overlap with the stdlib name "io".

Besides refactoring for its own sake, part of the goal is to get lib down the point where we can merge src/inference back into it; the split file makes it tough to track dependencies, and AFAICT it makes the move towards using cythonize easier.

[jbrockmendel]

Closing to declog CI pipeline.

[codecov]

Codecov Report

Merging #19364 into master will decrease coverage by <.01%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master   #19364      +/-   ##
==========================================
- Coverage   91.57%   91.56%   -0.01%     
==========================================
  Files         150      150              
  Lines       48705    48706       +1     
==========================================
- Hits        44600    44599       -1     
- Misses       4105     4107       +2

Flag	Coverage Δ
#multiple	89.93% <83.33%> (-0.01%)	⬇️
#single	41.72% <83.33%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/io/parsers.py	95.5% <100%> (ø)	⬆️
pandas/io/json/normalize.py	96.87% <100%> (ø)	⬆️
pandas/io/formats/format.py	96.24% <100%> (ø)	⬆️
pandas/io/pytables.py	92.45% <83.33%> (ø)	⬆️
pandas/util/testing.py	84.24% <0%> (-0.22%)	⬇️

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