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.

So, it is not an error if it returns "not found" due to the "design" of the legacy charm store?