[python] Factor io/ingest.py, create io/{_common,outgest}.py

[ryan-williams]

Issue and/or context: #2121

Changes:

• Move to_{h5ad,anndata} out to io/outgest.py
• Move constants used in both ingest and outgest to io/_common.py

Notes for Reviewer:

[johnkerl]

@ryan-williams looks good! Maybe rebase and re-run lint locally & re-push since #2274 is recently merged

[johnkerl]

@ryan-williams looks good! Maybe rebase and re-run lint locally & re-push since #2274 is recently merged

[codecov]

Codecov Report

Merging #2246 (24f1861) into main (874ccb2) will increase coverage by 12.27%.
The diff coverage is 86.76%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2246       +/-   ##
===========================================
+ Coverage   78.56%   90.84%   +12.27%     
===========================================
  Files         137       36      -101     
  Lines       10684     3855     -6829     
  Branches      215        0      -215     
===========================================
- Hits         8394     3502     -4892     
+ Misses       2192      353     -1839     
+ Partials       98        0       -98     

Flag	Coverage Δ
libtiledbsoma	?
python	90.84% <86.76%> (+0.66%)	⬆️
r	?

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Components	Coverage Δ
python_api	90.84% <86.76%> (+0.66%)	⬆️
libtiledbsoma	∅ <ø> (∅)