pub fn new(package_graph: &'g PackageGraph) -> Self {
        Self {
            package_graph,
            dfs_space: DfsSpace::new(&package_graph.dep_graph),
        }
    }
Creates a new cache for `depends_on` queries for this package graph.

This holds a shared reference to the package graph. This is to ensure that the cache is
invalidated if the package graph is mutated.
guppy/src/graph/graph_impl.rs