Call 'setup copy' and 'setup register' separately

We're working on this at ZuriHac. This change is a first step towards having more control over the installation process. Eventually, the actual copy and registration should be managed by `cabal-install` instead of `Setup.hs` so that we can:

 * Learn the installed package id of the package we just installed
 * Learn what files belong to a package and support uninstalling them
 * Sanity-check the installation before actually doing it