refactor(generate_classes): deprecate branch for ref, introduce repo, test commit hashes

[wpbonelli]

• add repo parameter, in case e.g. a fork is named differently
• add ref parameter, deprecate branch
  • should be generically named since supports hashes, branch names, tags
  • update usages in CI from branch to ref
• add tests for short- and long-form commit hashes
• update commit.yml and release.yml test jobs to use --dist loadfile
  • tests in test_generate_tests.py can't run in parallel due to pytest-virtualenv

[codecov]

Codecov Report

Merging #1907 (bd7d94b) into develop (8084b67) will decrease coverage by 0.1%.
Report is 4 commits behind head on develop.
The diff coverage is 8.3%.

❗ Current head bd7d94b differs from pull request most recent head ce681d6. Consider uploading reports for the commit ce681d6 to get more accurate results

@@            Coverage Diff            @@
##           develop   #1907     +/-   ##
=========================================
- Coverage     72.6%   72.5%   -0.1%     
=========================================
  Files          257     257             
  Lines        56234   56243      +9     
=========================================
+ Hits         40829   40830      +1     
- Misses       15405   15413      +8     

Files Changed	Coverage Δ
flopy/mf6/utils/generate_classes.py	16.8% <8.3%> (-0.6%)	⬇️

[wpbonelli]

possibly relevant to #1629

[mwtoews]

Looks good