Unnecessary dependency on `which`

Throughout the scripts and tests directory, this project uses the which executable. This is an external dependency and has a perfectly reasonable POSIX alternative, command -v. The which vs. command -v issue has recently caused a nontrivial amount of debate in the Debian community, and in the spirit of freedesktop.org's aim of interoperability, it would be nice to extricate which from xdg-utils.

Merge request to follow, but happy to discuss/make further changes if required.