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