custom go_package name, error

Describe the bug

When the go_package name is customized, the generated pb.fast.go is wrong

Steps to reproduce the behavior: custom go_package:

option go_package = "github.com/cloudwego/fastpb/examples/fastpb_gen/user/player;playercustom";

although the official documentation does not recommend this, but it is legal.

感谢反馈，我后续会看下这个问题以及你的pr。