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