HTools/Node: add mkNodeGraph function

This function helps treating node node problems as graph problems.
It can transform a list of nodes plus a list of instances into a graph
which uses the nodes as vertices, and instances as edges connecting them
(as long as they have both a primary and a secondary node)

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>