refactor: resolve ruff check F821 for undefined name

[mwtoews]

This is a minor refactor to resolve F821 for undefined name.

As with other checks, "flopy/mf6/**/*.py" is ignored (which currently has several F821 issues).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.9%. Comparing base (bb9824e) to head (54a2622).
Report is 7 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2374     +/-   ##
=========================================
+ Coverage     68.4%   75.9%   +7.5%     
=========================================
  Files          294     294             
  Lines        59390   61638   +2248     
=========================================
+ Hits         40652   46841   +6189     
+ Misses       18738   14797   -3941     

see 229 files with indirect coverage changes

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