graphs/depends: do not draw transitive dependencies by default

The transitive dependencies make the graphs barely readable for large
configs, with a large number of packages.

So, just switch to not drawing the transitive dependencies by default.

By popular demand... ;-)

[Peter: reword]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc; Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>