Allow composing from tag with unsigned packages
    
    There can be packages in the tag that will not end up in the compose.
    Instead of failing immediately with error, this patch delays the check
    until after depsolving finishes and only checks packages that will
    really be included.
    
    This is not an issue for nodeps compose, as that already pulls in only
    packages that will be composed and nothing else.
    
    Merges: https://pagure.io/pungi/pull-request/843
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>