Summary: Ref T9979. This is going to become `FulltextEngine`, but pave the way for that by pulling extensions out of it.
Test Plan:
{F1036624}
- Used `bin/search index Txxx`, saw projects, subscribers and custom fields rebuild in the index.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T9979
Differential Revision: https://secure.phabricator.com/D14835
