Move split_features() fn into crates_io_index crate

[Turbo87]

This fn is only relevant when writing out index files, so we can move it into our dedicated crates_io_index crate. This makes it faster to compile and test, since that crate compiles a lot faster than crates_io itself 😅

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.69%. Comparing base (43f3c91) to head (1c1a758).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9649   +/-   ##
=======================================
  Coverage   88.69%   88.69%           
=======================================
  Files         288      288           
  Lines       29366    29368    +2     
=======================================
+ Hits        26046    26048    +2     
  Misses       3320     3320           

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