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-ttomlhslujpx5sdpbu2c","name":"Foo","members":["PHID-USER-cb5af6p4oepy5tlgqypi"],"slugs":["foo","bar"],"dateCreated":"1402422720","dateModified":"1402422728"}},"slugMap":{"foo":"PHID-PROJ-ttomlhslujpx5sdpbu2c"},"cursor":{"limit":100,"after":null,"before":null}}}
```

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4418

Differential Revision: https://secure.phabricator.com/D9619