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Predicate download_plan/3
 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.