This means testing with local dev caches (not memcache) will warn
developers if they are introducing inadvertent importabilities. There is
also the ability to silence the warning if a dev is not planning to use
memcache and knows what they are doing with their keys.

Thanks to Carl Meyer for the patch. Fixed #6447.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13766 bcc190cf-cafb-0310-a4f2-bffc1f526a37