bpo-32030: Add Python/pathconfig.c (#4668)

* Factorize code from PC/getpathp.c and Modules/getpath.c to remove
  duplicated code
* rename pathconfig_clear() to _PyPathConfig_Clear()
* Inline _PyPathConfig_Fini() in pymain_impl() and then remove it,
  since it's a oneliner
10 files changed