ZSH Tab completion use virtualenv executables

[cbcoutinho]

• [x] I have searched the issues of this repo and believe that this is not a duplicate.
• [x] I have searched the documentation and believe that my question is not covered.

Feature Request

I commonly use poetry run <command> within a project, and it would be great if the zsh tab completion would pick up on all executables in $PATH, including the virtualenv, so that I could do:

$ poetry run py[TAB]
$ poetry run python
Python 3.7.6 (default, Jan 12 2020, 18:32:42)
[Clang 11.0.0 (clang-1100.0.33.16)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

The current implementation just completes poetry commands and files in the current directory.

[Secrus]

Poetry completions come from cleo (https://github.com/sdispater/cleo/blob/master/cleo/commands/completions/templates.py), so it should probably be upstreamed there.