Figure out how to deal with (re)exports

It would be nice to mark all the stuff the module exports (even if those are defined in an other module, and exported via Haskell's reexport functionality).

That would enable us to make Kythe-assisted auto-import tool.

Could make a Kythe semantic entity (abuse kind=interface?) to represent "module exports", add anchors childof that entity, with the anchors referencing ing the exported semantic nodes. + @creachadair