Fix zsh completion of `--pre-run` (#554)

* Fix zsh completion of `--pre-run`

Refer https://github.com/zsh-users/zsh/blob/master/Completion/Unix/Command/_sudo#L47-L51
the original completion of `--pre-run` will complete external commands, environment variables, built-in functions of zsh, etc.
Now it only complete external commands, which is expected result of `--pre-run`: it cannot run built-in functions of shell.

* Change pudb3 to pudb