refactor(mfstructure): add inspection utils, use for solution pkgs

[wpbonelli]

The MFStructure specification needs to know which packages are solutions. Find this out by reflection. This is a "wedge" to start deconstructing mfstructure.py and the same idea should work for other introspection cases e.g. subpackages, also sim_struct, though that will be more complicated.

Once #2333 merges we can add a dedicated class attribute to the template for this (and other attributes) and then read them off the class instead of parsing them from the dfn attribute.

[codecov]

Codecov Report

Attention: Patch coverage is 79.31034% with 6 lines in your changes missing coverage. Please review.

Project coverage is 74.6%. Comparing base (bb9824e) to head (b75d77b).
Report is 48 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/inspect.py	64.7%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2436     +/-   ##
=========================================
+ Coverage     68.4%   74.6%   +6.1%     
=========================================
  Files          294     295      +1     
  Lines        59390   60117    +727     
=========================================
+ Hits         40652   44858   +4206     
+ Misses       18738   15259   -3479     

Files with missing lines	Coverage Δ
flopy/mf6/data/mfstructure.py	64.4% <100.0%> (+0.3%)	⬆️
flopy/utils/inspect.py	100.0% <100.0%> (ø)
flopy/mf6/inspect.py	64.7% <64.7%> (ø)

... and 238 files with indirect coverage changes