gokrazy: use monorepo for gokrazy appliance builds (monogok)

This switches our gokrazy builds to use a new variant of cmd/gok called
opinionated about using monorepos: https://github.com/bradfitz/monogok

And with that, we can get rid of all the go.mod files and builddir forests
under gokrazy/**.

Updates #13038
Updates gokrazy/gokrazy#361

Change-Id: I9f18fbe59b8792286abc1e563d686ea9472c622d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

gokrazy/natlabapp/gokrazydeps.go

@@ -0,0 +1,16 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build for_go_mod_tidy_only
+
+package gokrazydeps
+
+import (
+	_ "github.com/gokrazy/gokrazy"
+	_ "github.com/gokrazy/gokrazy/cmd/dhcp"
+	_ "github.com/gokrazy/serial-busybox"
+	_ "github.com/tailscale/gokrazy-kernel"
+	_ "tailscale.com/cmd/tailscale"
+	_ "tailscale.com/cmd/tailscaled"
+	_ "tailscale.com/cmd/tta"
+)