Review Request: JDK-8155955: packager needs to determine the root modules to create JRE image

[Mandy Chung]

http://cr.openjdk.java.net/~mchung/jdk9/webrevs/8155955/webrev.00/

This provides some utility methods for packager to determine the platform modules to help packager independent of any API change.  This helper class will ultimately move to the packager implementation when FX upgrades its boot JDK later in JDK 9 release.  

Mandy