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>

feature/buildfeatures/feature_osrouter_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_osrouter
+
+package buildfeatures
+
+// HasOSRouter is whether the binary was built with support for modular feature "Configure the operating system's network stack, IPs, and routing tables".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_osrouter" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasOSRouter = false

feature/buildfeatures/feature_osrouter_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_osrouter
+
+package buildfeatures
+
+// HasOSRouter is whether the binary was built with support for modular feature "Configure the operating system's network stack, IPs, and routing tables".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_osrouter" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasOSRouter = true