bpo-36142: Exclude coreconfig.h from Py_LIMITED_API (GH-12111)

The whole coreconfig.h header is now excluded from Py_LIMITED_API.
Move functions definitions into a new internal pycore_coreconfig.h
header.

* Move Include/coreconfig.h to Include/cpython/coreconfig.h
* coreconfig.h header is now excluded from Py_LIMITED_API
* Move functions to pycore_coreconfig.h
diff --git a/Include/Python.h b/Include/Python.h
index aa60175..55b06ae 100644
--- a/Include/Python.h
+++ b/Include/Python.h
@@ -128,7 +128,7 @@
 #include "codecs.h"
 #include "pyerrors.h"
 
-#include "coreconfig.h"
+#include "cpython/coreconfig.h"
 #include "pystate.h"
 #include "context.h"