core/graph-depends: add option to graph reverse dependencies

core/graph-depends: add option to graph reverse dependencies

Now that we can dump the reverse dependencies of a package, add the
ability to graph those.

It does not make sense to do a full reverse graph, as it would be
semantically equivalent to the direct graph. So we only provide a
per-package reverse graph.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>