RFR: jmod remove

[Chris Hegarty]

I heard the following expression a few times, 'rm -rf has reached the 
limit of its usefulness'! So it's about time we had a 'jmod remove'.

The webrev is based upon Mandy's alias support patch [1]. I figured it 
would most likely be going in after the alias support.

http://cr.openjdk.java.net/~chegar/jigsaw/jmodrm.02/webrev/

Essentially, you can remove any module not required by a configuration. 
Fails if you try to remove a module which will break a configuration, 
unless you specify --force, or remove both the root module along with 
the dependent module.

I added a comment about the parent/child relationship. That is, a module 
library has no notion of its children so we could remove a module in a 
parent library that is not part of any configuration in that library, 
but may be part of a configuration in a child library. I think this is 
fine for now, but we may have to revisit the parent/child relationship 
in the future ( beyond the scope of jmod remove ).

-Chris.

[1] http://mail.openjdk.java.net/pipermail/jigsaw-dev/2012-April/002351.html