pub fn package_count(&self) -> usize {
        // This can be obtained in two different ways: self.dep_graph.node_count() or
        // self.data.packages.len(). verify() checks that they return the same results.
        //
        // Use this way for symmetry with link_count below (which can only be obtained through the
        // graph).
        self.dep_graph.node_count()
    }
Returns the number of packages in this graph.
guppy/src/graph/graph_impl.rs