Reorganize code

[adrhill]

• Split package extension tests into their own test files to facilitate future PRs from external collaborators
• Split manual overloads into separate files
• Rename source files with outdated names

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.36%. Comparing base (8f2a318) to head (db6a873).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #179      +/-   ##
==========================================
+ Coverage   91.28%   91.36%   +0.07%     
==========================================
  Files          37       41       +4     
  Lines        1779     1772       -7     
==========================================
- Hits         1624     1619       -5     
+ Misses        155      153       -2     

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