pub fn link_between(
        &self,
        other: &PackageId,
        direction: DependencyDirection,
    ) -> Result<Option<PackageLink<'g>>, Error> {
        self.link_between_impl(other, direction.into())
    }
Returns the direct `PackageLink` between `self` and `other` in the specified direction:
* `Forward`: from `self` to `other`
* `Reverse`: from `other` to `self`

Returns `None` if the direct link does not exist, or an error if `to` isn't found in
`self.graph()`.
guppy/src/graph/graph_impl.rs