Move tag inheritance check into a separate method
    
    Tag inheritance check may not be required by all downstream package
    tools built on top of rpkg. Moving this check into a separate method
    would be easy for downstream tools to customize the behavior. By
    default, tag inheritance check happens when doing a chain build. Once
    some tool does not need the check, it can be disabled in derived
    Commands class.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>