depmod: handle nested loops

depmod: handle nested loops

This is a rework of depmod report cycles logic to make it
tolerant to more complex loops.

The patch tries to remember own path for vertexes which makes it
possible to handle configurations with common edges and
non-cyclic modules.

It assumes that the previous dependency calculations can not give
as input something like

mod_a -> mod_b -> <loop>, but

<loop> -> mod_a -> mod_b should be fine.

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>