lppublish: migrate bundles from legacy format

It doesn't provide the bundle's charms for the migration process yet,
as we need charm archive PUT for the charm revisions to work
correctly with respect to the charm URLs mentioned in the bundles.

Manually verified that at least some bundles get correctly imported.
What if we try to publish as much bundles as possible and not just return on the first error? On the one hand, we get all the errors on one ingestion, on the other hand, it does nothing to actually solve the error of the bundle. Not a blocker or something we need right away, just a thought.