Don't depend on "XMLWriter" to load the lint renderer class tree

Summary: Fixes T13489. See that task for discussion.

Test Plan: Ran arc lint and arc lint --output xml. Faked a missing extension, ran arc lint (no problems) and arc lint --output xml (sensible error message).

Maniphest Tasks: T13489

Differential Revision: https://secure.phabricator.com/D20989