Reduce method visibility.

Summary: These methods are declared public, but there are meant to be protected (as they are declared in ArcanistExternalLinter).

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8991