cipd: Make it work on Windows.

go/src/infra/tools/cipd/local/rename_posix.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // +build !windows
 
-package main
+package local
 
 import (
-        "syscall"
+        "os"
 )
 
-var serverStartParams = &syscall.SysProcAttr{Setpgid: true}
+func atomicRename(source, target string) error {
+        return os.Rename(source, target)
+}