dockerTools.tarsum: Fix upstream import

pkgs/build-support/docker/default.nix

@@ -44,8 +44,8 @@ rec {
 
     cp ${./tarsum.go} tarsum.go
     export GOPATH=$(pwd)
-    mkdir src
-    ln -sT ${docker.src}/components/engine/pkg/tarsum src/tarsum
+    mkdir -p src/github.com/docker/docker/pkg
+    ln -sT ${docker.src}/components/engine/pkg/tarsum src/github.com/docker/docker/pkg/tarsum
     go build
 
     cp tarsum $out

pkgs/build-support/docker/tarsum.go

@@ -5,7 +5,7 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
-	"tarsum"
+	"github.com/docker/docker/pkg/tarsum"
 )
 
 func main() {