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.
The getCharm part is needed because the new bundle spec requires relation endpoints to be specified completely, including the relation name. Previously they were optional. The only way of finding out the intended relation names is by fetching the charms themselves and resolving the implied relation names. The fetcher will be implemented after the PUT request has been implemented, so that there's some hope of charm URLs resolving correctly.