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.

cmd/frpc/main.go

@@ -1,31 +0,0 @@
-package main
-
-import (
-	"os"
-	"sync"
-
-	"github.com/fatedier/frp/models/client"
-	"github.com/fatedier/frp/utils/log"
-)
-
-func main() {
-	err := client.LoadConf("./frpc.ini")
-	if err != nil {
-		os.Exit(-1)
-	}
-
-	log.InitLog(client.LogWay, client.LogFile, client.LogLevel)
-
-	// wait until all control goroutine exit
-	var wait sync.WaitGroup
-	wait.Add(len(client.ProxyClients))
-
-	for _, client := range client.ProxyClients {
-		go ControlProcess(client, &wait)
-	}
-
-	log.Info("Start frpc success")
-
-	wait.Wait()
-	log.Warn("All proxy exit!")
-}