RFR: 8209003: Consolidate use of empty collections in java.lang.module

[Claes Redestad]

Hi,

feel free to review this cleanup to consolidate some uses of 
Collections.empty/unmodifiableXx.. to Set/Map/List.of()/copyOf(), as 
appropriate. This aligns better with code already generated by jlink, so 
avoids observing a mix of many implementation classes in some places..

Webrev: http://cr.openjdk.java.net/~redestad/8209003/jdk.00/

Bug: https://bugs.openjdk.java.net/browse/JDK-8209003

Due the specified need to throw IAE rather than NPE in certain cases, 
code in some performance-insensitive API methods in 
ModuleDescriptor.Builder now does an extra copy up front 
(List/Set/Map.copyOf will throw NPE on any null element or key). I'd 
rather keep the code simple than add extra handling to squeeze out the 
last bit of performance here (maybe I'm getting old...)

Thanks!

/Claes