Don't use modules

Feel free to revert this if you have a strong feeling about it. But so
far as I can see, it adds a lot of complexity for basically no upsides.

device.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"git.zx2c4.com/wireguard-go/internal/ratelimiter"
 	"runtime"
 	"sync"
 	"sync/atomic"
@@ -51,7 +50,7 @@ type Device struct {
 
 	rate struct {
 		underLoadUntil atomic.Value
-		limiter        ratelimiter.Ratelimiter
+		limiter        Ratelimiter
 	}
 
 	pool struct {