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.
I'd probably write the latter, but OTH I like what you did as it is "independent" from actual members of the struct.