python module: produce the correct install path on every OS

[eli-schwartz]

The sysconfig paths are, by default, correct for every OS -- they are supposed to follow the scheme that python knows about per default.

For some reason, this overrode the scheme to posix_prefix, which is the default for posix OSes like linux and macOS, but wrong on Windows. Simply deleting this entirely makes everything that used to work, still work, and a couple new things start working.

@MathieuDuponchelle I'm seeking insight into why this originally got implemented the way it did. Do you happen to remember?

Fixes #3995

[codecov]

Codecov Report

Merging #9156 (688f176) into master (0063eb2) will decrease coverage by 3.17%.
The diff coverage is n/a.

❗ Current head 688f176 differs from pull request most recent head f87ada2. Consider uploading reports for the commit f87ada2 to get more accurate results

@@            Coverage Diff             @@
##           master    #9156      +/-   ##
==========================================
- Coverage   66.69%   63.52%   -3.18%     
==========================================
  Files         378      189     -189     
  Lines       84510    42192   -42318     
  Branches    17491     8733    -8758     
==========================================
- Hits        56368    26802   -29566     
+ Misses      23349    13038   -10311     
+ Partials     4793     2352    -2441     

Impacted Files	Coverage Δ
mesonbuild/compilers/mixins/clang.py
mesonbuild/dependencies/hdf5.py
mesonbuild/backend/backends.py
mesonbuild/interpreter/compiler.py
mesonbuild/interpreter/__init__.py
mesonbuild/cmake/generator.py
mesonbuild/interpreter/interpreterobjects.py
mesonbuild/compilers/mixins/gnu.py
mesonbuild/modules/unstable_simd.py
mesonbuild/dependencies/cmake.py
... and 179 more

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[xclaesse]

I'm wondering if we should go one step further on Windows. By default sys.path has C:\Python38\lib\site-packages but with this PR meson is going to install into <prefix>\lib\site-packages which by default is C:\lib\site-packages. Note sure what should be the best heuristic here, if meson prefix is the root of the same drive where python is installed, then we use the platlib without overriding 'base': ''?

[xclaesse]

otoh, this is always going to be at best a guess. Maybe what we really need is adding an option, we discussed recently we should allow modules to add their own options. Currently every project that install python modules pretty much have to add their own option which makes it inconsistent.

[MathieuDuponchelle]

@eli-schwartz I remember writing this specific line, but I really don't remember the reason, should have added an inline comment probably :(

[eli-schwartz]

Forgot that this isn't merged yet... Note this has been discussed previously in #3995 which this would fix.