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.
copy works on slices, but this isn't doing something on a slice.

I swithered a bit between this and:

    finalURLs[i] = &charm.Reference{
       Name: url.Name + "-" + name,
       User: url.User,
       Revision: url.Revision,
       Series: url.Series,
   }

I'm still not entirely sure I chose the right way.