Don't lint /webroot/rsrc/externals/.

Summary: If I understand correctly, the /webroot/rsrc/externals/ directory should be treated similarly to /externals/ in terms of linting and coding style.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9689