Add internal routine _PyModule_Clear(), which does approximately what

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).