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/refactor/importgraph/graph.go b/refactor/importgraph/graph.go
index bbf8879..bbdbb33 100644
--- a/refactor/importgraph/graph.go
+++ b/refactor/importgraph/graph.go
@@ -10,7 +10,7 @@
 	"go/build"
 	"sync"
 
-	"code.google.com/p/go.tools/go/buildutil"
+	"golang.org/x/tools/go/buildutil"
 )
 
 // A Graph is an import dependency graph, either forward or reverse.