Function ensure_command_on_path 

pub fn ensure_command_on_path() -> Outcome

Make the managed pair reachable from a fresh shell.

This is the step that lets pip install dev-prune in a virtualenv survive the virtualenv: the binaries pip placed vanish with the environment, but the managed copy under <config>/bin does not, and after this step it is the one a new terminal finds. See crate::pathenv for what “reachable” means per platform.