Fixture lints should encourage directory organization

[baileyparker]

As the course progresses and more compiler phases are developed, the fixtures directory will quickly become unwieldy. As a mitigation to this, a new linter should be created that encourages using directories. For example, for a fixture with only a .scanner phase it should recommend moving the .sim and .scanner file to fixtures/scanner/. For a fixture with a .scanner and .cst phase, it should recommend moving the .sim, .scanner, and .cst to fixtures/cst.