- download_plan(+Targets, +Plan, +Options) is semidet[private]
- Download or update all packages from Plan. We need to do this as a
first step because we may not have (up-to-date) dependency
information about all packs. For example, a pack may be installed at
the git HEAD revision that is not yet known to the server or it may
be installed from a url that is not known at all at the server.