feat: use go:embed to embed templates (#1756)

tools/goctl/api/gogen/genmain.go

@@ -1,6 +1,7 @@
 package gogen
 
 import (
+	_ "embed"
 	"fmt"
 	"strings"
 
@@ -11,33 +12,8 @@ import (
 	"github.com/zeromicro/go-zero/tools/goctl/vars"
 )
 
-const mainTemplate = `package main
-
-import (
-	"flag"
-	"fmt"
-
-	{{.importPackages}}
-)
-
-var configFile = flag.String("f", "etc/{{.serviceName}}.yaml", "the config file")
-
-func main() {
-	flag.Parse()
-
-	var c config.Config
-	conf.MustLoad(*configFile, &c)
-
-	ctx := svc.NewServiceContext(c)
-	server := rest.MustNewServer(c.RestConf)
-	defer server.Stop()
-
-	handler.RegisterHandlers(server, ctx)
-
-	fmt.Printf("Starting server at %s:%d...\n", c.Host, c.Port)
-	server.Start()
-}
-`
+//go:embed main.tpl
+var mainTemplate string
 
 func genMain(dir, rootPkg string, cfg *config.Config, api *spec.ApiSpec) error {
 	name := strings.ToLower(api.Service.Name)