Fixing so that on windows local paths to binaries can be used

Summary: On windows, using something like "vendor/bin/phpcs" does not work because the code tries to run "where vendor/bin/phpcs" which fails badly. So instead, when we try to test for a binary on windows, we first check if a file exists locally with that name. If it does, just use that. If someone has a file in their project with the same name as a linter, but that is not the actual linter, this could fail, but I can't imagine a case where that would happen.

Test Plan: On windows, run `arc lint` with a linter configured that is local e.g. "vendor/bin/phpcs"

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9493