gather: Add fulltree-exclude flag to DNF backend
    
    This is needed for correct trimming of addons and optional. A package
    pulled in as a dependency but that matches something on fulltree exclude
    list should have this flag. It will then be moved from addon to base
    variant and therefore excluded from optional.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>