go: x/tools/cmd/guru: describe: objects defined in cgo files are not found

golang guru/GoDef cannot find non-golang package documentation that is in the GOPATH.

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?

   go version go1.6.1 linux/386

2. What operating system and processor architecture are you using (go env)?

   GOARCH="386"
   GOBIN=""
   GOEXE=""
   GOHOSTARCH="386"
   GOHOSTOS="linux"
   GOOS="linux"
   GOPATH="/home/erik/go:/opt/erik/dev/g/go:/opt/erik/dev/p/go"
   GORACE=""
   GOROOT="/usr/lib/go"
   GOTOOLDIR="/usr/lib/go/pkg/tool/linux_386"
   GO15VENDOREXPERIMENT="1"
   CC="gcc"
   GOGCCFLAGS="-fPIC -m32 -pthread -fmessage-length=0"
   CXX="g++"
   CGO_ENABLED="1"
   

3. What did you do? I filed an issue with vim-go. (https://github.com/fatih/vim-go/issues/851#issuecomment-219248856). The vim-go author believes the problem is with golang guru. Please see the issue for the repeatable problem.

4. What did you expect to see?

Please see the issue above.

1. What did you see instead?

Please see the issue above.