pub(super) fn add_nodes(&mut self, package: PackageMetadata<'_>) {
        let base_node = FeatureNode::base(package.package_ix());
        let base_ix = self.add_node(base_node);
        self.base_ixs.push(base_ix);
        FeatureNode::named_features(package)
            .chain(FeatureNode::optional_deps(package))
            .for_each(|feature_node| {
                let feature_ix = self.add_node(feature_node);
                self.graph
                    .update_edge(feature_ix, base_ix, FeatureEdge::FeatureToBase);
            });
    }
Add nodes for every feature in this package + the base package, and add edges from every
feature to the base package.
guppy/src/graph/feature/build.rs