net/packet: split off checksum munging into different pkg

The current structure meant that we were embedding netstack in
the tailscale CLI and in the GUIs. This removes that by isolating
the checksum munging to a different pkg which is only called from
`net/tstun`.

Fixes #9756

Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/tailscale/tailscale_test.go

@@ -0,0 +1,21 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"testing"
+
+	"tailscale.com/tstest/deptest"
+)
+
+func TestDeps(t *testing.T) {
+	deptest.DepChecker{
+		BadDeps: map[string]string{
+			"gvisor.dev/gvisor/pkg/buffer":       "https://github.com/tailscale/tailscale/issues/9756",
+			"gvisor.dev/gvisor/pkg/cpuid":        "https://github.com/tailscale/tailscale/issues/9756",
+			"gvisor.dev/gvisor/pkg/tcpip":        "https://github.com/tailscale/tailscale/issues/9756",
+			"gvisor.dev/gvisor/pkg/tcpip/header": "https://github.com/tailscale/tailscale/issues/9756",
+		},
+	}.Check(t)
+}