gather: Process dependencies sorted
    
    Instead of going in random order, we should do it deterministically and
    process the dependencies in order.
    
    This should actually fix a problem where `glibc-langpack` packages are
    missing on some architectures for Fedora Rawhide.
    
    Fixes: https://pagure.io/pungi-fedora/issue/214
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>