Data cleanup: private submodules, rename AttributeSet -> EntityAttribute, type cleanup

[mdekstrand]

This closes #946 and advances #947 by cleaning up the lenskit.data module, making submodules private and renaming AttributeSet to EntityAttribute.

Also includes various other type cleanups.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.45%. Comparing base (b148ed0) to head (6c3ec66).
⚠️ Report is 37 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1067      +/-   ##
==========================================
- Coverage   89.49%   89.45%   -0.04%     
==========================================
  Files         233      233              
  Lines       15890    15870      -20     
==========================================
- Hits        14220    14197      -23     
- Misses       1670     1673       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.