Add internal routine _PyModule_Clear(), which does approximately what

clear_carefully() used to do in import.c.  Differences: leave only
__builtins__ alone in the 2nd pass; and don't clear the dictionary (on
the theory that as long as there are references left to the
dictionary, those might be destructors that might expect __builtins__
to be alive when they run; and __builtins__ can't normally be part of
a cycle).

Include/moduleobject.h

@@ -44,6 +44,7 @@ extern DL_IMPORT(PyTypeObject) PyModule_Type;
 extern PyObject *PyModule_New Py_PROTO((char *));
 extern PyObject *PyModule_GetDict Py_PROTO((PyObject *));
 extern char *PyModule_GetName Py_PROTO((PyObject *));
+extern void _PyModule_Clear Py_PROTO((PyObject *));
 
 #ifdef __cplusplus
 }