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/consts/consts.go

@@ -0,0 +1,13 @@
+package consts
+
+// server status
+const (
+	Idle = iota
+	Working
+)
+
+// connection type
+const (
+	CtlConn = iota
+	WorkConn
+)