lppublish, cmd/charmload: ingest exact revisions from legacy charm store

This behaviour is controlled by the new "-legacy" flag to charmload, which should
be used just once, when initially uploading charms.

In passing, also adjusted log messages somewhat so the logging at INFO
level is a little quieter and more informative.

Good question.

Actually, it shouldn't return a not-found error at all for the bulk meta endpoint.
The returned map should just omit entries that were not found.
I'll remove this logic in a subsequent branch.