go/ast: adjustments to filter function

CL 4938041 made some incorrect changes to the filter
function which caused a different doc/codelab/wiki/index.html
file to be generated.

Added FilterFileExports and FilterPackageExports function.
Same as the existing FileExpors/PackageExports functions
but using shared code. The old functions will be removed
in the next CL.

R=r, rsc
CC=golang-dev
https://golang.org/cl/4932048