implement _index_data parts of #24024

[jbrockmendel]

Besides narrowing down the scope of #24024, I think this might fix the segfaults I had in my old DTA branch, which may make it feasible to hugely simplify the composition changeover.

Aside from removing apparently-unused imports in reduction.pyx, the affected code is copied verbatim from #24024.

[pep8speaks]

Hello @jbrockmendel! Thanks for submitting the PR.

• There are no PEP8 issues in the file pandas/core/indexes/base.py !

• There are no PEP8 issues in the file pandas/core/indexes/datetimes.py !

• There are no PEP8 issues in the file pandas/core/indexes/period.py !

• There are no PEP8 issues in the file pandas/core/indexes/timedeltas.py !

[codecov]

Codecov Report

Merging #24379 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #24379      +/-   ##
==========================================
+ Coverage   92.29%   92.29%   +<.01%     
==========================================
  Files         162      162              
  Lines       51841    51845       +4     
==========================================
+ Hits        47847    47852       +5     
+ Misses       3994     3993       -1

Flag	Coverage Δ
#multiple	90.7% <100%> (ø)	⬆️
#single	42.98% <100%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/period.py	93.11% <100%> (+0.01%)	⬆️
pandas/core/indexes/datetimes.py	96.33% <100%> (ø)	⬆️
pandas/core/indexes/base.py	96.27% <100%> (ø)	⬆️
pandas/core/indexes/timedeltas.py	90.44% <100%> (+0.02%)	⬆️
pandas/util/testing.py	87.66% <0%> (+0.09%)	⬆️

---

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[codecov]

Codecov Report

Merging #24379 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #24379      +/-   ##
==========================================
+ Coverage   92.29%   92.29%   +<.01%     
==========================================
  Files         162      162              
  Lines       51841    51845       +4     
==========================================
+ Hits        47847    47852       +5     
+ Misses       3994     3993       -1

Flag	Coverage Δ
#multiple	90.7% <100%> (ø)	⬆️
#single	42.98% <100%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/period.py	93.11% <100%> (+0.01%)	⬆️
pandas/core/indexes/datetimes.py	96.33% <100%> (ø)	⬆️
pandas/core/indexes/base.py	96.27% <100%> (ø)	⬆️
pandas/core/indexes/timedeltas.py	90.44% <100%> (+0.02%)	⬆️
pandas/util/testing.py	87.66% <0%> (+0.09%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5d134ec...350b0ec. Read the comment docs.

[jreback]

thanks!