support/graph-depends: use argparse to parse argv[]

Currently, we are using a crude, ad-hoc parsing of argv[].
This is a limiting factor to adding new options.

Use argparse instead, and introduce a single argument for now:
  --package, -p PACKAGE

In the (near) future, we'll be able to add more option arguments,
such as depth-limiting for big graphs.

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