support/graph-depends: allow excluding virtual packages

Like for --stop-on, make --exclude recognise the keyword 'virtual',
to stop on virtual packages (as explained in the help...).

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