[WIP] bpo-42260: Rewrite getpath.c in Python

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• Add _getpath Python module and _cgetpath C module
• Py_SetPath() no longer computes the path configuration.

https://bugs.python.org/issue42260

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The PR is still a draft.

Modules/getpath.c is reimplemented in pure Python as Lib/_getpath.py. The module is frozen by "make regen-importlib" and run by by pyinit_main():

PyRun_SimpleString("import _getpath; _getpath.main()");

I kept Modules/getpath.c to ease development, but it will be fully removed in the final patch.

There are likely bugs in my conversion from getpath.c (C) to _getpath.py (Python).

The new Python module gets a companion _cgetpath: a C extension to expose functions needed by _getpath (Python).

Code size: 1623 lines for getpath.c (C) => 654 lines for _getpath.py (Python), and way simpler string manipulation: 2.5x smaller code.

Right now, 4 tests are still failing (test_embed, test_sys, test_gdb and a 4th that I forgot!).

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cc @ncoghlan @zooba

See also https://www.python.org/dev/peps/pep-0432/

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Remaining issue: test_init_read_set() fails. The error on Windows is different than the error on Linux/macOS.

Windows:

-  'base_executable': 'D:\\a\\cpython\\cpython\\PCbuild\\amd64\\_testembed.exe',
+  'base_executable': 'my_executable',

Linux/macOS:

   'module_search_paths': ['/opt/python-dev/lib/python310.zip',
-                          'Lib',
?                           ^ ^

+                          'test_path_insert1',
?                           ^^^^^^^^^^ ^^^^^^

+                          '/Users/runner/work/cpython/cpython/Lib',
-                          'build/lib.macosx-10.15-x86_64-3.10-pydebug'],
?                                                                      -

+                          '/Users/runner/work/cpython/cpython/build/lib.macosx-10.15-x86_64-3.10-pydebug',
?                           +++++++++++++++++++++++++++++++++++

+                          'test_path_append'],

[github-actions]

This PR is stale because it has been open for 30 days with no activity.