support/graph-depends: add option to stop on specific packages

Add a new option to graph-depends, that users can set to stop the graph
on a specific (set of) package(s).

This accepts any actual package name, or the 'virtual' keyword to stop
on virtual packages.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Francois Perrad <fperrad@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>