Fixed #23670 -- Prevented partial import state during module autodiscovery

Thanks kostko for the report.
Fixed #23715 -- Prevented urlize from treating a trailing ! as part of an URL

Thanks to 57even for the report.
Fixed #23468 -- settings.FIXTURE_DIRS checks in loaddata command

If settings.FIXTURE_DIRS contains duplicates or a default fixture
directory (app_name/fixtures), ImproperlyConfigured is raised.

Thanks to Berker Peksag and Tim Graham for review.