cmd/tailscaled: split package main into main shim + package

So we can empty import the guts of cmd/tailscaled from another
module for go mod tidy reasons.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/tailscaled-main.go

@@ -0,0 +1,19 @@
+// Copyright (c) 2021 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.
+
+// The tailscaled program is the Tailscale daemon. It's configured
+// and controlled via either the tailscale CLI program or GUIs.
+package main // import "tailscale.com/cmd/tailscaled"
+
+import "tailscale.com/cmd/tailscaled/tailscaled"
+
+// Don't add any new imports or code to this file. The real
+// code is in tailscale.com/cmd/tailscale/tailscaled as a package
+// so things can depend on it for dependency reasons.
+// (Go programs can't import package main so we split the real code
+// off where we could have a dummy package empty import it)
+
+func main() {
+	tailscaled.Main()
+}