pub fn metadata(&self, package_id: &PackageId) -> Result<PackageMetadata, Error> {
        let inner = self
            .data
            .metadata_impl(package_id)
            .ok_or_else(|| Error::UnknownPackageId(package_id.clone()))?;
        Ok(PackageMetadata::new(self, inner))
    }
Returns the metadata for the given package ID.
guppy/src/graph/graph_impl.rs