Add a visibility annotation that means "visible in this package and its subpackages"

As requested at https://groups.google.com/d/topic/closure-compiler-discuss/x37vv_TAg1g/discussion

The idea is an annotation that allows a variable/property to be accessed from other code in the same package _or subpackages_ but not anywhere else. I'm not sure what the annotation would be. `@subpackages` perhaps.