raco pkg update: make `--lookup` and `--clone` work right together

The combination of `--lookup`, `--clone`, and `--catalog` can provide
a one-step path from a snapshot's built implementation of a package to
a repository-linked clone, for example.  In that situation, however,
`raco pkg` will have limited ability to detect that packages
originally drawn from the same repository are consistenly installed as
clones (and we can revisit if it turns out to be an issue).