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
diff --git a/Makefile.pre.in b/Makefile.pre.in
index f425a89..14f6f8a 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -337,8 +337,9 @@
 		Python/importdl.o \
 		Python/marshal.o \
 		Python/modsupport.o \
-		Python/mystrtoul.o \
 		Python/mysnprintf.o \
+		Python/mystrtoul.o \
+		Python/pathconfig.o \
 		Python/peephole.o \
 		Python/pyarena.o \
 		Python/pyctype.o \