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

[Tim Bell]

Erik:

> 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/

Looks good to me.

/Tim