pub fn depends_on(&self, package_a: &PackageId, package_b: &PackageId) -> Result<bool, Error> {
        let mut depends_cache = self.new_depends_cache();
        depends_cache.depends_on(package_a, package_b)
    }
Returns true if `package_a` depends (directly or indirectly) on `package_b`.

In other words, this returns true if `package_b` is a (possibly transitive) dependency of
`package_a`.

This also returns true if `package_a` is the same as `package_b`.

For repeated queries, consider using `new_depends_cache` to speed up queries.
guppy/src/graph/graph_impl.rs