Only collect tasks from installed apps

[PeterJCLaw]

While tasks files are currently only explicitly imported from installed apps, tasks (and their queues) are implicitly registered when other apps are imported. This can mean that non-installed apps end up having their queues registered and extra workers created. For a large codebase with lots of optional apps this can represent quite a lot of redundant workers.

It would be great if there was a way around this. Maybe some form of check in that the queue comes from a task which is in an installed app?