bpo-32030: Cleanup "path config" code (#4663)

* Rename PyPathConfig structure to _PyPathConfig and move it to
  Include/internal/pystate.h
* Rename path_config to _Py_path_config
* _PyPathConfig: Rename program_name field to program_full_path
* Add assert(str != NULL); to _PyMem_RawWcsdup(), _PyMem_RawStrdup()
  and _PyMem_Strdup().
* Rename calculate_path() to pathconfig_global_init(). The function
  now does nothing if it's already initiallized.