checkmappings enhancements

[JamiesWhiteShirt]

The checkMappings command currently only checks the mappings for package access violations. Here is a list of things I believe we should be able to detect and report to ensure the integrity of mappings:

1. Package access violations. It is ready for testing.
2. Method naming conflicts. Enigma makes these hard to create, but there are alternative mapping tools and a potential for people using text editors for small edits, creating room for error. We recently had issues in yarn because a mapping was carelessly changed in a text editor causing a conflict.
3. Dropped/unnecessary mappings. Enigma likes to drop mappings that are no longer needed and we should be able to check for those.