Modularize "arc lint" renderers

Summary:
Ref T12996. arc lint can render in different formats with --output <format>.

Previously, these were a big switch() statement and a bunch of hard-coded stuff.

Modularize them so that that third parties could add new renderers, and this code can be refactored toward parallelizing the lint step.

This has a small behavioral change: we no longer hide "autofix" messages. I'd like to generally simplify the number of amend/autofix flags here, so this edges us toward that slightly.

Test Plan: Ran arc lint with all the --output flags in states with warnings and no warnings, saw sensible-seeming behavior.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12996

Differential Revision: https://secure.phabricator.com/D18645