RFR: JDK-8153969: Clean up module src dir logic

[Erik Joelsson]

We currently have 3 places in the makefiles where the full list of 
source dirs for each module is calculated. I have refactored this into 
macros in make/common/Modules.gmk so that it's handled uniformly. This 
will also make it simpler for other potential users of this data, like 
IDE project generation.

Bug: https://bugs.openjdk.java.net/browse/JDK-8153969
Webrev: http://cr.openjdk.java.net/~erikj/8153969/webrev.top.01/
/Erik