Function forbid_path 

pub fn forbid_path(
    graph: &Graph,
    from: &str,
    to: &str,
    allowed: &HashSet<EdgeKind>,
) -> Result<Option<Violation>, String>

Evaluate --forbid 'A=>B': a violation when any package named from reaches a package named to over allowed edges. Err when from is not in the graph at all (a defective assertion, not a clean pass).