Add tooldeps package to keep depaware pinned in go.mod.

internal/tooldeps/tooldeps.go

@@ -0,0 +1,9 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tooldeps
+
+import (
+	_ "github.com/tailscale/depaware/depaware"
+)