Avoid loading autoloaded file

[syohex]

In zsh file name convention, file name of which starts with '_' is an autoloaded file. It should not be loaded manually.

This fixes https://github.com/zsh-users/zsh-completions/issues/809

[jandamm]

I've fixed this issue at zgenom. Your solution prohibits the user to manually load a file which starts with _. zgen load <plugin> path/to/_file.sh would not source this file.