Thanks Florian for isolating the shortest way to reproduce this issue:
./runtests.py \
django.contrib.auth.tests.test_context_processors.AuthContextProcessorTests.test_perms_attrs \
django.contrib.auth.tests.test_auth_backends.ChangedBackendSettingsTest.test_changed_backend_settings \
django.contrib.auth.tests.test_auth_backends.CustomUserModelBackendAuthenticateTest.test_authenticate \
django.contrib.auth.tests.test_basic.BasicTestCase.test_createsuperuser_management_command
The package renaming restores the older package names (which were also the
documented package names). This doesn't affect test discovery because the
module in question doesn't contain any tests.
Thanks to Carl for the design discussion.