fix(resolve_exe): support extensionless abs/rel paths on windows

[wpbonelli]

• close bug: location of mf6 executable using relative path #1633
• followup to fix(exe path): FloPy now correctly resolves relative paths to mf6 executable (#1633) #1727

The motivation here is symmetry of portability — relative paths with .exe suffix will work on non-Windows, and paths without .exe suffix will work on Windows, provided the relpath is otherwise identical

[codecov]

Codecov Report

Merging #1957 (b24e38d) into develop (9e9d730) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##           develop   #1957   +/-   ##
=======================================
  Coverage     72.6%   72.6%           
=======================================
  Files          257     257           
  Lines        57812   57818    +6     
=======================================
+ Hits         42009   42029   +20     
+ Misses       15803   15789   -14     

Files Changed	Coverage Δ
flopy/mbase.py	70.0% <100.0%> (+0.7%)	⬆️

... and 3 files with indirect coverage changes

[wpbonelli]

after reconsidering the comment here

could easily be added for the next minor version release

IMO this is more of a fix than a feature so we might wrap it into 3.4.3 (planned for this weekend)

[wpbonelli]

@langevin-usgs it would be good to get this into 3.4.3. CI will fail on develop after merging, but it shouldn't affect the release as bugfix commits will be cherry-picked

[langevin-usgs]

Maybe this is end of this issue? Thanks, @wpbonelli.