Move Phabricator import workers to correct location

These should be in app/workers. If they aren't, they won't be picked up by every_sidekiq_worker_spec.rb, and their queues won't be guaranteed to be in all_queues.yml, which in turn prevents sidekiq-cluster from seeing them.

Making the BaseWorker not include ApplicationWorker means that we don't need to act as if it has its own queue. (See HashedStorage::BaseWorker, which does the same thing.)