go.tools: use golang.org/x/... import paths

Rewrite performed with this command:
  sed -i '' 's_code.google.com/p/go\._golang.org/x/_g' \
    $(grep -lr 'code.google.com/p/go.' *)

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/170920043
diff --git a/godoc/index_test.go b/godoc/index_test.go
index 562c97a..a16bdcb 100644
--- a/godoc/index_test.go
+++ b/godoc/index_test.go
@@ -11,7 +11,7 @@
 	"strings"
 	"testing"
 
-	"code.google.com/p/go.tools/godoc/vfs/mapfs"
+	"golang.org/x/tools/godoc/vfs/mapfs"
 )
 
 func newCorpus(t *testing.T) *Corpus {