refactor: use pathlib for the paths pymake builds

[jdhughes-dev]

os.path is replaced by pathlib where a path is built or a file is removed in pymake.py, which is 15 of the 28 calls.

The rest are left because os.path does something pathlib does not, and each was checked rather than assumed:

	os.path	pathlib
normpath('a/../b')	b	a/../b, the parent directories are kept
splitext('bin/mf6.exe')[0]	bin/mf6	.stem is mf6, the directory is dropped
dirname('mf6')	empty	.
abspath	leaves a symbolic link	.resolve() follows it

normpath tidies the extrafiles paths that are written to a build file, dirname is compared with appdir to decide where a target is written, and abspath decides whether a target has been built already, so converting any of them changes what pymake does.

Checked with the 67 base tests, by writing the mfusg makefile and comparing it, and by building triangle into an appdir, which is what the dirname comparison decides.

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.170%. Comparing base (0c765d3) to head (c14bc0f).

Files with missing lines	Patch %	Lines
pymake/pymake.py	57.142%	6 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           develop      #336       +/-   ##
=============================================
- Coverage   80.231%   80.170%   -0.061%     
=============================================
  Files           20        20               
  Lines         3283      3283               
=============================================
- Hits          2634      2632        -2     
- Misses         649       651        +2     

Files with missing lines	Coverage Δ
pymake/pymake.py	75.659% <57.142%> (ø)

... and 1 file with indirect coverage changes

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