Module library attributes

[Sean Mullan]

I'd like to be able to store a persistent attribute with a module when it is
installed that could then be retrieved at runtime when all I have is a reference
to the Module object. I don't think this is quite the same as
metadata, as listed in the jigsaw requirements. It is more like data that we can
use to optimize our implementation. It doesn't necessarily have to be exposed
via a public API.

--Sean