Grapher extension does not deal with PrivateModules

_From [mail4danny](https://code.google.com/u/104267830025841335740/) on June 07, 2010 14:07:01_

When composing an application using PrivateModules, the grapher extension
ignores non-exposed bindings. Specifically,
GraphingVisitor.visit(ExposedBinding<?> binding) is a no-op, and
TransitiveDependencyVisitor.visit<ExposedBinding<?> binding> returns an
empty set of dependencies.

I've had a go at fixing this, and I'm sorry to say I got a bit lost! :-(

What would be awesome would be if the graphing output grouped bindings from
particular modules in, say, a big dashed-line box. (Actually, that would be
awesome regardless of the PrivateModule issue!)

_Original issue: http://code.google.com/p/google-guice/issues/detail?id=489_