Summary:
This moves them out of linters since we both have lint and "unit test"
engines in there.
Cleans up the organization of all the subfolders and makes the naming a bit more
consistent.
Adds a missing License file.

Test Plan: `arc lint` and `arc unit` should continue to work.

Reviewers: zasgar, michelle

Reviewed By: zasgar

Differential Revision: https://phab.corp.pixielabs.ai/D8105

GitOrigin-RevId: 9677abf9f9b3e7f460413ba54fdaeac5d55a431b