Consolidate repository identity resolution and detection code

Summary: Ref T13444. Send all repository identity/detection through a new "DiffusionRepositoryIdentityEngine" which handles resolution and detection updates in one place.

Test Plan:

• Ran bin/repository reparse --message ..., saw author/committer identity updates.
• Added "goose@example.com" to my email addresses, ran daemons, saw the identity relationship get picked up.
• Ran bin/repository rebuild-identities ..., saw sensible rebuilds.

Maniphest Tasks: T13444

Differential Revision: https://secure.phabricator.com/D20910