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/router_windows_test.go

@@ -0,0 +1,19 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package osrouter
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+func TestGetNetshPath(t *testing.T) {
+	ft := &firewallTweaker{
+		logf: t.Logf,
+	}
+	path := ft.getNetshPath()
+	if !filepath.IsAbs(path) {
+		t.Errorf("expected absolute path for netsh.exe: %q", path)
+	}
+}