R1:396433672237

Include project slugs in the results of a `project.query` Conduit call.

Summary: Ref T4418. This feature will be used by D9457 to determine whether the specified slugs exist.

Test Plan:
Made a conduit call with `arc`:

```
> echo '{"slugs": ["foo"]}' | arc --conduit-uri='http://phabricator.joshuaspence.com' call-conduit project.query
Waiting for JSON parameters on stdin...
{"error":null,"errorMessage":null,"response":{"data":{"PHID-PROJ-ttomlhslujpx5sdpbu2c":{"id":"1","phid":"PHID-PROJ…