Detect definitions used but not reexported

[pjljvandelaar]

Will weeder 2.0 support the detection of not reexported definitions?
See ndmitchell/weeder#48 for more info.

[ocharles]

This would be easier for me to answer if I had some kind of minimal example. Could you sketch out some Haskell files, and mark which things you think should be weeds? It doesn't even necessarily have to compile, but I have a better time understanding if I can see example code.