wgengine/router{,/osrouter}: split OS router implementations into subpackage

So wgengine/router is just the docs + entrypoint + types, and then
underscore importing wgengine/router/osrouter registers the constructors
with the wgengine/router package.

Then tsnet can not pull those in.

Updates #17313

Change-Id: If313226f6987d709ea9193c8f16a909326ceefe7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/router/osrouter/osrouter.go

@@ -0,0 +1,15 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package osrouter contains OS-specific router implementations.
+// This package has no API; it exists purely to import
+// for the side effect of it registering itself with the wgengine/router
+// package.
+package osrouter
+
+import "tailscale.com/wgengine/router"
+
+// shutdownConfig is a routing configuration that removes all router
+// state from the OS. It's the config used when callers pass in a nil
+// Config.
+var shutdownConfig router.Config