fix(MFFileMgmt): fix strip_model_relative_path to avoid IndexError

[wpbonelli]

• Fix Load simulation error "list index out of range" #1747. MFSimulation.load() with absolute or relative paths on Windows could raise IndexError, tracing to MFFileMgmt.strip_model_relative_path. Avoid this and refactor to use pathlib. Test providing namefile by name, by absolute path, and by relative path

• Since Modflow6 supports POSIX paths even on Windows, use forward slash separators in relative path strings returned by strip_model_relative_path on all OS — previously used backslashes on Windows

• Convert back- to forward-slashes in filename ctor argument to MFPackage. Fixes an error introduced in fix(MFFileMgmt): remove string_to_file_path #1759 occurring with a relative filename with Windows separators

• Currently FloPy MFSimulation.write_simulation() and similar still emit backslashes on Windows, in future these could write posix paths too. Add a test case for this but ignore it for now

[codecov]

Codecov Report

Merging #1748 (cd64b6d) into develop (6041835) will increase coverage by 0.1%.
The diff coverage is 100.0%.

❗ Current head cd64b6d differs from pull request most recent head 0d611fc. Consider uploading reports for the commit 0d611fc to get more accurate results

@@            Coverage Diff            @@
##           develop   #1748     +/-   ##
=========================================
+ Coverage     72.0%   72.2%   +0.1%     
=========================================
  Files          254     253      -1     
  Lines        55915   56057    +142     
=========================================
+ Hits         40302   40496    +194     
+ Misses       15613   15561     -52     

Impacted Files	Coverage Δ
flopy/mf6/mfbase.py	81.5% <100.0%> (+0.2%)	⬆️
flopy/mf6/mfpackage.py	66.2% <100.0%> (+0.9%)	⬆️

... and 21 files with indirect coverage changes

[langevin-usgs]

Any thoughts @spaulins-usgs?

[wpbonelli]

Aside from the MFFileMgmt patch, this PR fixes the regression test failing since #1759 (filename param on MFPackage should support names or rel path, provided as str or PathLike). This could move to its own PR if that would be cleaner.