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

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