Make splitting submodules private

[mdekstrand]

This advances #947 by making the submodules of lenskit.splitting private.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.45%. Comparing base (6f33824) to head (8b95fc9).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1068   +/-   ##
=======================================
  Coverage   89.45%   89.45%           
=======================================
  Files         233      233           
  Lines       15870    15870           
=======================================
  Hits        14197    14197           
  Misses       1673     1673           

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