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_test.go

@@ -0,0 +1,15 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package osrouter
+
+import "net/netip"
+
+//lint:ignore U1000 used in Windows/Linux tests only
+func mustCIDRs(ss ...string) []netip.Prefix {
+	var ret []netip.Prefix
+	for _, s := range ss {
+		ret = append(ret, netip.MustParsePrefix(s))
+	}
+	return ret
+}