Improved detection on Linux.

[oboecat]

At the moment when looking for commands on Linux, it executes echo $(which -a cmd1; which -a cmd2; ...etc), which outputs the paths separated by spaces instead of \n.
Instead of executing multiple instances of which, my changes pass which all the commands that need to be queried. This is probably more stable.

[sindresorhus]

Are you sure which -a always outputs on the same line? At least on macOS, which is BSD based, it outputs on separate lines.

[sindresorhus]

This also needs to handle if the command doesn't exist.

❯ which -a foo bar
foo not found
bar not found

[sindresorhus]

Closing because of #42