all: improve the method of import for internal packages

1. Change directory structure and Makefile to let GOPATH=`pwd`, so wherever the project directory is, just use make to build.

src/frp/models/msg/msg.go

@@ -0,0 +1,20 @@
+package msg
+
+type GeneralRes struct {
+	Code int64  `json:"code"`
+	Msg  string `json:"msg"`
+}
+
+type ClientCtlReq struct {
+	Type      int64  `json:"type"`
+	ProxyName string `json:"proxy_name"`
+	Passwd    string `json:"passwd"`
+}
+
+type ClientCtlRes struct {
+	GeneralRes
+}
+
+type ServerCtlReq struct {
+	Type int64 `json:"type"`
+}