cmd/godoc: fix directory read

cmd/godoc: fix directory read

Undo CL 5783076 and apply correct fix.

The /doc hack is wrong.  The code to handle this case was
already there and just needs a simple fix:

 	// We didn't find any directories containing Go files.
 	// If some directory returned successfully, use that.
-	if len(all) == 0 && first != nil {
+	if !haveGo {
 		for _, d := range first {
 			haveName[d.Name()] = true
 			all = append(all, d)

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5783079