[amberctl] switch amberctl to use pkg resolver for caching packages

This switches amberctl to use package resolver for caching packages,
instead of talking directly to amber, which will eventually make it
easier to remove amber without disruption.

Note that this also removes `amberctl get_up -nowait ...`, because
that functionality cannot be replicated with the PackageResolver API
and golang's synchronous implementation of FIDL.

Test: Manually confirmed it can cache packages.
Bug: PKG-520 #done
Change-Id: I4e0b878bf3b31edfe7d0fe23afc080e26c0e7e04