RFR: JDK-8055922: Work around sjavac limitation with public api tracking cross modules

[Erik Joelsson]

Hello,

Please review this little workaround for a current shortcoming in 
Sjavac. See bug for more details. With this change, Sjavac will start 
acting correctly again and not miss any files that need to be 
recompiled. The approximation is course however, we should still fix 
https://bugs.openjdk.java.net/browse/JDK-8054717 to properly only 
recompile what is really necessary. Still, this workaround does add 
value compared to running without sjavac. With this change and 
JDK-8014510 (also on review) I'm starting to feel confident enough to 
make sjavac default for building jdk9.

Bug: https://bugs.openjdk.java.net/browse/JDK-8055922
Webrev: http://cr.openjdk.java.net/~erikj/8055922/webrev.root.01/

/Erik