refactor(codegen): move dfn utils from devtools

[wpbonelli]

To avoid the dependency on a development branch of modflow-devtools, copy over the DFN parser, TOML conversion script, etc for now. Also include the necessary transitive dependencies. Workaround for 3.9.3's failure to launch.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 320 lines in your changes missing coverage. Please review.

Project coverage is 55.8%. Comparing base (7eb7c88) to head (ebe68bb).
Report is 16 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/utils/dfn.py	0.0%	292 Missing ⚠️
flopy/mf6/utils/dfn2toml.py	0.0%	24 Missing ⚠️
flopy/mf6/utils/codegen/__init__.py	0.0%	1 Missing ⚠️
flopy/mf6/utils/codegen/component.py	0.0%	1 Missing ⚠️
flopy/mf6/utils/codegen/filters.py	0.0%	1 Missing ⚠️
flopy/mf6/utils/generate_classes.py	0.0%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2508      +/-   ##
===========================================
- Coverage     76.1%    55.8%   -20.3%     
===========================================
  Files          296      644     +348     
  Lines        64215   125001   +60786     
===========================================
+ Hits         48885    69787   +20902     
- Misses       15330    55214   +39884     

Files with missing lines	Coverage Δ
flopy/mf6/utils/codegen/__init__.py	0.0% <0.0%> (ø)
flopy/mf6/utils/codegen/component.py	0.0% <0.0%> (ø)
flopy/mf6/utils/codegen/filters.py	0.0% <0.0%> (ø)
flopy/mf6/utils/generate_classes.py	18.6% <0.0%> (ø)
flopy/mf6/utils/dfn2toml.py	0.0% <0.0%> (ø)
flopy/mf6/utils/dfn.py	0.0% <0.0%> (ø)

... and 477 files with indirect coverage changes

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