Module command-not-found: Added command-not-found hook for Arch Linux

Arch Linux also has a command-not-found tool: https://wiki.archlinux.org/index.php/Pkgfile
I've changed the command-not-found module so that the pkgfile hook gets sourced when it is found. :)

This is my first GitHub pull request, so please tell me if I made a mistake or should improve the code.