Having a replace directive in go.mod causes "go get
github.com/rclone/rclone" to fail as it discussed in this Go issue:
https://github.com/golang/go/issues/44840
This is apparently how the Go team want go.mod to work, so this commit
hard forks github.com/jlaffaye/ftp into github.com/rclone/ftp so we
can remove the `replace` directive from the go.mod file.
Fixes#5810