8150668: Layer.defineModulesXXX with a Configuration containing java.base throws undocumented exception

[Alan Bateman]

This is another small fix where the Layer javadoc doesn't make it clear 
that there can only be one "java.base" module in the VM. The enforcement 
has been there but it wasn't specified and there weren't tests for this 
scenario.
     http://cr.openjdk.java.net/~alanb/8150668/webrev/

Thanks,
Alan.