pub fn to_feature_set(&self, filter: impl FeatureFilter<'g>) -> FeatureSet<'g> {
        let feature_graph = self.graph.feature_graph();
        let included: IxBitSet = feature_graph.feature_ixs_for_package_ixs_filtered(
            // The direction of iteration doesn't matter.
            self.ixs(DependencyDirection::Forward),
            filter,
        );
        FeatureSet::from_included(feature_graph, included)
    }
Creates a new `FeatureSet` consisting of all packages in this `PackageSet`, using the given
feature filter.

This will cause the feature graph to be constructed if it hasn't been done so already.
guppy/src/graph/resolve.rs