Expose symbol information over Conduit

Summary: I want to add a command like "where is ArcanistUnitTestEngine" to
phabot. I also want to add a symbol typeahead to Diffusion and generally finish
up that feature since it's useful but only half-implemented. Consolidate the
query logic and expose the data over Conduit.

Test Plan: Used /symbol/ and Conduit to lookup symbols.

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: aran, jungejason

Maniphest Tasks: T315

Differential Revision: https://secure.phabricator.com/D1260