pub fn feature_graph(&self) -> FeatureGraph {
        let inner = self.get_feature_graph();
        FeatureGraph {
            package_graph: self,
            inner,
        }
    }
Returns a derived graph representing every feature of every package.

The feature graph is constructed the first time this method is called. The graph is cached
so that repeated calls to this method are cheap.
guppy/src/graph/feature/graph_impl.rs