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.
Would it make sense to prepend the README with a note about auto-generated bundle and all the bundles that were generated with this one?