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