Add unit tests for ApplicationValidator checks

[wildmaples]

We have ApplicationValidatorTest which are integration tests. However, we should find a way to unit test the individual checks.

We could make unit testing this file easier by refactoring it so a fake filesystem can be injected.
This would make it easier to test but would require a larger refactor. https://github.com/Shopify/packwerk-old/pull/262#discussion_r455956863

TO DO:

• check_autoload_path_cache,
• check_package_manifests_for_privacy,
• check_package_manifest_syntax,
• check_application_structure,
• check_inflection_file,
• check_acyclic_graph,
• check_package_manifest_paths,
• check_valid_package_dependencies,
• check_root_package_exist,

[rochlefebvre]

Addressed in #84