RFR 8168925: MODULES property should be topologically ordered and space-separated list

[Sundararajan Athijegannathan]

Please review 
http://cr.openjdk.java.net/~sundar/8168925/webrev.01/index.html for 
https://bugs.openjdk.java.net/browse/JDK-8168925

PS. Mandy Chung wrote ModuleSorter for another fix (yet to be pushed).  
I'm using it for this fix after discussion with her (private email).

Thanks,
-Sundar