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/consolidating_router_test.go

@@ -4,7 +4,6 @@
 package router
 
 import (
-	"log"
 	"net/netip"
 	"testing"
 
@@ -56,7 +55,7 @@ func TestConsolidateRoutes(t *testing.T) {
 		},
 	}
 
-	cr := &consolidatingRouter{logf: log.Printf}
+	cr := &consolidatingRouter{logf: t.Logf}
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
 			got := cr.consolidateRoutes(test.cfg)