Review Request: JDK-8173374: Update GenGraphs tool to generate dot graph with requires transitive edges

[Mandy Chung]

http://cr.openjdk.java.net/~mchung/jdk9/webrevs/8173374/webrev.00/

This is the first step to enable generating dot graph to be 
included in module summary javadoc, if desired.  jdeps already
supports generating the dot graph for modules.  This patch
converts GenGraphs build tool to use jdeps implementation
as well as fixes jdeps to work with -apionly to generate
a dot graph containing `requires transitive` edges only.

Mandy
[1] https://bugs.openjdk.java.net/browse/JDK-8173303