[1.2.X] Fixed #14698 -- Ensure that module_has_sumodule doesn't mistake a...

[1.2.X] 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.

Backport of r15362 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15364 bcc190cf-cafb-0310-a4f2-bffc1f526a37