Move more discovery responsibilities into DiscoveryEngine

Summary:
Ref T4327. This moves the last pieces of discovery responsibility out of the PullLocal daemon and into the DiscoveryEngine.

(This makes it easier to discover repositories in unit tests in the future, since we don't need to build a PullLocal daemon and can just build a DiscoveryEngine.)

Test Plan: Ran `phd debug pulllocal`. Ran `repostory discover`.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4327

Differential Revision: https://secure.phabricator.com/D7997