modules without classes

[Alan Bateman]

I was chatting with Peter Jensen about a problem he's seeing trying to 
install some of our current jdk* modules that that only has a 
module-info. It turns out the recent refactoring of the module 
reader/writer code breaks this case as the index creation assumes that 
the there are classes in the module directory.  This is no longer the 
case when installing a jmod file that doesn't have a CLASSES section. 
I'll sort this out and add a test so that we catch this scenario but it 
begs the question as to whether it make sense to have an empty classes 
in the module directory or not? I think it should be okay to not require 
it (if for no other reason that many zip tools don't tolerate empty zip 
files anyway).

-Alan.