R1:254f606e8973

Tie all the pieces for symbol cross-references together

Summary:
This makes symbol cross-references work in Differential. You need to do a little
legwork but I'll document that once the change has baked for a little while.

Basically:

- Projects are annotated with indexed languages, and "shared library" projects
(for example, symbols in Phabricator should be searched for in Arcanist and
libphutil).
- When we render a changeset, we check if its language is an indexed one. If
it is, we invoke the…