When loading or reloading an extension, the main extension module's

.admin submodule was being imported or reloaded. If the extension had

apps with their own .admin, those would never be reloaded, even though

Django itself would initially import them. This would cause admin site

registrations to disappear on extension reload, since the AdminSite

would be thrown away and the new one would never be set up.
We now properly handle an import/reload for each extension app's

.admin submodule.
Note that if an extension was trying to import one .admin from another

to work around this, they might get duplicate registration errors that

prevent the extension from loading. However, these extensions would have

had problems during reload anyway.

Unit tests passed.
Triggered frequent extension state reloading and verified an issue I hit

with Power Pack's admin registrations went away, and that the appropriate

modules were reloading.