bpo-38353: Cleanup includes in the internal C API (GH-16548)

Use forward declaration of types to avoid includes in the internal C
API. Add also comment to justify other includes.
diff --git a/PC/getpathp.c b/PC/getpathp.c
index 04f24d9..b17c002 100644
--- a/PC/getpathp.c
+++ b/PC/getpathp.c
@@ -80,7 +80,8 @@
 
 
 #include "Python.h"
-#include "pycore_initconfig.h"
+#include "pycore_initconfig.h"   /* PyStatus */
+#include "pycore_pathconfig.h"   /* _PyPathConfig */
 #include "pycore_pystate.h"
 #include "osdefs.h"
 #include <wchar.h>