pub fn is_cyclic(&self, a: &PackageId, b: &PackageId) -> Result<bool, Error> {
        let a_ix = self.package_graph.package_ix(a)?;
        let b_ix = self.package_graph.package_ix(b)?;
        Ok(self.sccs.is_same_scc(a_ix, b_ix))
    }
Returns true if these two IDs are in the same cycle.
guppy/src/graph/cycles.rs