Support Checkstyle as an output format for lint results.

Summary: Fixes T4948. Add a lint renderer which supports outputting the lint results in the Checkstyle XML format.

Test Plan: Ran `arc lint --xml` and inspected the output.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4948

Differential Revision: https://secure.phabricator.com/D9083