Stop using deprecated Conduit methods in `arc`

Summary:
  - Replace `maniphest.find` with `maniphest.query`. These calls are nearly identical, it was just a rename for consistency.
  - Replace `differential.find` with `differential.query`.

Test Plan:
  - Ran `arc tasks`.
  - Ran `arc close-revision` on valid, nonexistent, and existent-but-invalid revisions.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6336