graphs: add option to remove transitive dependencies in dependency graph

Currently, all the dependencies of a package are drawn on the dependency
graph, including transitive dependencies (e.g. A->B->C and A->C).

For very big graphs, with lots of packages with lots of dependencies, the
dependency graph can be very dense, and transitive dependencies are
cluttering the graph.

In some cases, only getting the "build-order" dependencies is enough (e.g.
to see what impact a package rebuild would have).

Add a new environment variable to disable drawing transitive dependencies.

Basically, it would turn this graph:

    pkg1 ---> pkg2 ---> pkg3 -------------------.
         |\__________/                 \         \
         |\____________________         \         \
         |                     \         \         \
          `-> pkg4 ---> pkg5 ---> pkg6 ---> pkg7 ---> pkg8
                    \__________/

into that graph:

    pkg1 ---> pkg2 ---> pkg3 -----------.
         |                               \
          `-> pkg4 ---> pkg5 ---> pkg6 ---> pkg7 ---> pkg8

[Thanks to Samuel for the parser hints]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>