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.
Could you make a quick explanation why Migrate call is designed as it is, please? (esp. regarding the getCharm part of the Migrate parameters)