Fixed #8485: Reordered imports in the database backends to ensure that the...

Fixed #8485: Reordered imports in the database backends to ensure that the availability of the MySQL and Oracle backends is confirmed before any attempt is made to use the backend. Many thanks to Karen Tracey for her work in narrowing down a number of vague reports into a concrete problem.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8466 bcc190cf-cafb-0310-a4f2-bffc1f526a37