move udp packet to protocol

common/protocol/udp/packet.go

@@ -0,0 +1,13 @@
+package udp
+
+import (
+	"v2ray.com/core/common/buf"
+	"v2ray.com/core/common/net"
+)
+
+// Packet is a UDP packet together with its source and destination address.
+type Packet struct {
+	Payload *buf.Buffer
+	Source  net.Destination
+	Target  net.Destination
+}

common/protocol/udp/udp.go

@@ -0,0 +1 @@
+package udp