This is the last piece of Clayton's #26179 to be implemented with file tags.
All diffs are accounted for. Followup will use this to streamline some
packages.
Also add some V(5) debugging - it was helpful in diagnosing various issues, it
may be helpful again.
His PR cam during the middle of this development cycle, and it was easier to
burn it down and recreate it than try to patch it into an existing series and
re-test every assumption. This behavior will be re-introduced in subsequent
commits.
Downstream generators that want to reuse the upstream generated types
need to be able to define a different ignore tag (so that they can see
the already generated types).
