Fixed #14698 -- Ensure that module_has_sumodule doesn't mistake a cache miss for an existent package. Thanks to Łukasz Rekucki for the report and patch, and to shields for the test case.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15362 bcc190cf-cafb-0310-a4f2-bffc1f526a37