Summary:
These were extending `ArcanistLinter` instead of `ArcanistExternalLinter`
Cleanup futures and lint message collection too.

Test Plan:
Introduce a variety of errors that would be caught by golint/govet/goimports.
Run `arc lint --trace`

Reviewers: zasgar

Reviewed By: zasgar

Differential Revision: https://phab.corp.pixielabs.ai/D7716

GitOrigin-RevId: 555e0a1d65198fa4918d24895988047c7ebf74bd