bpo-45211: Move helpers from getpath.c to internal API. (gh-28550)

This accomplishes 2 things:

* consolidates some common code between getpath.c and getpathp.c
* makes the helpers available to code in other files

FWIW, the signature of the join_relfile() function (in fileutils.c) intentionally mirrors that of Windows' PathCchCombineEx().

Note that this change is mostly moving code around. No behavior is meant to change.

https://bugs.python.org/issue45211

Lib/test/test_embed.py

@@ -434,7 +434,7 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
         'pathconfig_warnings': 1,
         '_init_main': 1,
         '_isolated_interpreter': 0,
-        'use_frozen_modules': False,
+        'use_frozen_modules': 0,
     }
     if MS_WINDOWS:
         CONFIG_COMPAT.update({