Removed ad-hoc support for usage of short names of built-in DB backends.

        # listing all possible (built-in) database backends.

        backend_dir = os.path.join(os.path.dirname(__file__), 'backends')

        try:

            available_backends = [f for f in os.listdir(backend_dir)

            builtin_backends = [f for f in os.listdir(backend_dir)

                    if os.path.isdir(os.path.join(backend_dir, f))

                    and not (f.startswith('.') or f == '__pycache__')]

                    and not (f.startswith('.') or f in ('__pycache__', 'dummy'))]

        except EnvironmentError:

            available_backends = []

        full_notation = backend_name.startswith('django.db.backends.')

        if full_notation:

            backend_name = backend_name[19:] # See #15621.

        if backend_name not in available_backends:

            backend_reprs = map(repr, sorted(available_backends))

            builtin_backends = []

        if backend_name not in ['django.db.backends.%s' % b for b in

                                builtin_backends]:

            backend_reprs = map(repr, sorted(builtin_backends))

            error_msg = ("%r isn't an available database backend.\n"

                         "Try using django.db.backends.XXX, where XXX "

                         "Try using 'django.db.backends.XXX', where XXX "

                         "is one of:\n    %s\nError was: %s" %

                         (backend_name, ", ".join(backend_reprs), e_user))

            raise ImproperlyConfigured(error_msg)

        elif not full_notation:

            # user tried to use the old notation for the database backend

            error_msg = ("%r isn't an available database backend.\n"

                         "Try using django.db.backends.%s instead.\n"

                         "Error was: %s" %

                         (backend_name, backend_name, e_user))

            raise ImproperlyConfigured(error_msg)

        else:

            # If there's some other error, this must be an error in Django

            raise

        self.assertEqual(len(exceptions), 0)

class BackendLoadingTests(TestCase):

    def test_old_style_backends_raise_useful_exception(self):

        six.assertRaisesRegex(self, ImproperlyConfigured,

            "Try using django.db.backends.sqlite3 instead",

            load_backend, 'sqlite3')

class MySQLPKZeroTests(TestCase):

    """

    Zero as id for AutoField should raise exception in MySQL, because MySQL