pub fn directly_depends_on(
        &self,
        package_a: &PackageId,
        package_b: &PackageId,
    ) -> Result<bool, Error> {
        let a_ix = self.package_ix(package_a)?;
        let b_ix = self.package_ix(package_b)?;
        Ok(self.dep_graph.contains_edge(a_ix, b_ix))
    }
Returns true if `package_a` directly depends on `package_b`.

In other words, this returns true if `package_b` is a direct dependency of `package_a`.

This returns false if `package_a` is the same as `package_b`.
guppy/src/graph/graph_impl.rs