cannot find package

```
go run main.go -u=https://xxx.m3u8  -o=~/Downloads/xxx.mp4  

main.go:8:2: cannot find package "github.com/oopsguy/m3u8/dl" in any of:
	/usr/local/Cellar/go/1.15.1/libexec/src/github.com/oopsguy/m3u8/dl (from $GOROOT)
	/Users/alpha/go/src/github.com/oopsguy/m3u8/dl (from $GOPATH)
```

git clone的代码, 在项目根目录运行, 报这个错