When running against a file, it should check the files that it links to as well

If checksync is run against a.js, and a.js has a tag pointing to b.go, checksync should expand to also validate that the tags that b.go contains that point back to a.js are also correct.

This is really what folks are going to expect and this enables the use of checksync to only validate changed files, while catching sync-tags broken by those changed files.