compose image/tree: support base image as lockfile

[cgwalters]

We should support something like
rpm-ostree compose image|tree --from-locked quay.io/centos/centos-bootable-base:stream9 or whatever.

This would use an existing image as basically a lockfile - we will depsolve to the exact same versions of the packages from that image by default (unless explicitly overridden).

I guess we want both --from-locked and --from which would actually inherit the package set too.