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