Fail early if input packages are unsigned
    
    Use 'get_packages_to_gather' to fail early if these packages are not
    signed with right key. This prevents us from having to wait for the
    repo to be created and depsolving to finish. Unsigned dependencies will
    still be reported later than previously.
    
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>