gather: Resolve weak dependencies
    
    This patch updates depsolving via DNF to treat Recommends the same as
    Requires, and adds a separate step to pull in packages that Supplement
    something that is already in the compose.
    
    This is all hidden behind a configuration flag. The default behavior is
    not changed.
    
    Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1569242
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>