pub fn features_for(&self, package_id: &PackageId) -> Result<Option<FeatureList<'g>>, Error> {
        let package = self.graph.package_graph.metadata(package_id)?;
        Ok(self.features_for_package_impl(package))
    }
Returns a list of features present for this package, or `None` if this package is not
present in the feature set.

Returns an error if the package ID was unknown.
guppy/src/graph/feature/resolve.rs