* With 160 envs, cuts time from ~14s to ~2.5s
* Change the output to be more like `pyenv-versions'
* Shellcheck fixes
* virtualenv: Only show non-envs for completion since env engines don't support piggy-backing envs
* Remove dead/unused code
* Fix IFS handling
`IFS=: <command>' doesn't apply that IFS to expansions in <command>,
it has to be set separately earlier
* Adjust and refactor tests
---------
Co-authored-by: Ivan Pozdeev <vano@mail.mipt.ru>
conda's environment might not have `python` executable. If the prefix
doesn't contain `python` in it, `pyenv-which` might be ran into infinite
loop if some of `which` hooks invoke `pyenv-virtualenv-prefix`.