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