graph-depends: display virtual package with italic style

virtual packages are found by their version,
so we retrieve the version of all packages

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>