graphs: store and keep intermediate 'dot' program for graph-depends

Currently, graph-depends (and PKG-graph-depends) do not store the
intermediate 'dot' program.

Some users would like to get the dot program to be able to further
customise the generated graphs (eg. modify the layout, colorise some
of the packages...)

So, store the intermediate dot program alongside the generated graph.

Reported-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Maxime Hadjinlian" <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>