- exports.go contains a stripped-down (but semantically unchanged)
  version of the code in go/ast/filter.go for export filtering
- filter.go contains the documentation filtering code found before
  at the end of doc.go; this is simply a code move w/o any semantic
  changes
- godoc now relies on go/doc for export filtering when creating
  documentation. It still has a separate form of export filtering
  for showing the source code version. This needs to be consolidated
  (perhaps the source form view should just be removed?).
- Stripping of function bodies (stripFunctionBodies function of
  godoc.go) is now happening in doc.go (line 176).
- doc.NewPackageDoc has an extra parameter "exportsOnly. If set
  to false, the behavior is as before. This function is only called
  once in our source code; a gofix module is probably not warranted.
- Deleted doc.NewFileDoc - was never called.

This change is mostly a code move w/ some minimal tweaks. It should
not cause any changes to the behavior of godoc. It's a prerequisite
for extracting anonymous embedded fields.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5502072