bpo-35081: Add _PyCoreConfig_AsDict() (GH-10362)

_testcapimodule.c must not include pycore_pathconfig.h, since it's an
internal header files.

Changes:

* Add _PyCoreConfig_AsDict() function to coreconfig.c.
* Remove pycore_pathconfig.h include from _testcapimodule.h.
* pycore_pathconfig.h now requires Py_BUILD_CORE to be defined.
* _testcapimodule.c compilation now fails if it's built with
  Py_BUILD_CORE defined.

Include/coreconfig.h

@@ -358,10 +358,11 @@ PyAPI_FUNC(int) _PyCoreConfig_GetEnvDup(
     wchar_t **dest,
     wchar_t *wname,
     char *name);
+
+/* Used by _testcapi.get_coreconfig() */
+PyAPI_FUNC(PyObject *) _PyCoreConfig_AsDict(const _PyCoreConfig *config);
 #endif
 
-
-
 #ifdef __cplusplus
 }
 #endif

Include/internal/pycore_pathconfig.h

@@ -4,6 +4,10 @@
 extern "C" {
 #endif
 
+#ifndef Py_BUILD_CORE
+#  error "Py_BUILD_CORE must be defined to include this header"
+#endif
+
 PyAPI_FUNC(void) _Py_wstrlist_clear(
     int len,
     wchar_t **list);