some further catalog/uniload cleanup

- The checkout catalog.uniload now does not uniload a resolver, and
  instead just has a trivial implementation of resolveConstraints.
  (The 'built' catalog.uniload already didn't use the resolver.)

- catalog.complete.resolveConstraints now throws an error if there's
  no resolver; this is OK, because creating the resolver now only
  uses the distinct catalog.uniload, so there's no recursion issue.

- don't record version dependencies in packages during uniload (this
  protects against using release.current before it is initialized)

- PackageLoader should never download packages during uniload (this
  protects against using catalog.uniload.isLocalPackage before it is
  initialized)