Fix the command checks in the debian plugin

The previous implementation depended on the assumption that `which` would return a path when `sudo` and/or `aptitude` were installed; however, when a command is aliased, e.g., `alias sudo='nocorrect sudo'`, no path is returned. This caused a false negative on my box.

This may not be the best way to correct the issue. I am still new to zsh scripting and shell scripting in general.