gather: Use arch packages in nodeps method
    
    We already filtered a list of packages compatible with the architecture.
    There's no need to do that again (with bugs). Instead of using the
    global package set we should just restrict the code to an arch package
    set.
    
    This should not break anything. If a package is included such that it's
    in global but not arch package set, the compose would crash as linking
    packages takes paths from the arch package set.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>