Add support for "harbormaster.target.search"

Summary: Ref T13222. See PHI986. See PHI896. Harbormaster build targets don't currently have a modern "*.search" API, but there's no reason not to provide one (even if some of the use cases are a little bit questionable).

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19841