Bundle validation: verify with charms.

Validate the bundle with respect to charms in the store.
I'm not sure about charm.Reference here. How about using *charm.Reference; that
way you can associate the exact *charm.Reference stored in the urls slice
with the string that created it.

Alternatively (and slightly more efficient) you could just have a slice
to go alongside the urls slice:

  idKeys := make([]string, 0, len(ids))

then append the original form of each id to it when you
append to urls.

On balance, I think I prefer the latter.