Use Go1.11 module (#5743)

* Migrate to go modules

* make vendor

* Update mvdan.cc/xurls

* make vendor

* Update code.gitea.io/git

* make fmt-check

* Update github.com/go-sql-driver/mysql

* make vendor

vendor/google.golang.org/appengine/internal/identity.go

@@ -0,0 +1,14 @@
+// Copyright 2011 Google Inc. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package internal
+
+import netcontext "golang.org/x/net/context"
+
+// These functions are implementations of the wrapper functions
+// in ../appengine/identity.go. See that file for commentary.
+
+func AppID(c netcontext.Context) string {
+	return appID(FullyQualifiedAppID(c))
+}