Modernize `ArcanistScalaSBTLinter`.

Summary: Ref T2039. Convert the `ArcanistScalaSBTLinter` into an `ArcanistExternalLinter` and make it compatible with `.arclint`.

Test Plan: I can't really test this because I don't have any Scala projects and we don't have any test cases.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: codeblock, epriestley, Korvin

Maniphest Tasks: T2039

Differential Revision: https://secure.phabricator.com/D9097