graph-depends: split off get_rdepends from get_depends

graph-depends: split off get_rdepends from get_depends

The use of a 'rule' variable that can contain 'show-depends' or
'show-rdepends' is not logical if get_depends is considered as a reusable
function from various scripts. The name of these rules are too much an
implementation detail.

Therefore, split the existing get_depends into two separate functions
get_depends and get_rdepends, while keeping code duplication to a minimum.

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