pub fn link_from(&self, from: &PackageId) -> Result<Option<PackageLink<'g>>, Error> {
        self.link_between_impl(from, Incoming)
    }
Returns the direct `PackageLink` from the specified package to `self`, or `None` if the
specified package does not directly depend on `self`.

Returns an error if `from` isn't found in `self.graph()`.
guppy/src/graph/graph_impl.rs