completions: support generation when running as module #231

When generating completions, we use the following logic to infer the command name:

As stated, this doesn't work when running the script as a module via python -m:

$ poetry completions zsh | head -1
#compdef poetry
$ python -m poetry completions zsh | head -1
#compdef __main__.py

We should fix this

Note: I noticed this since Arch Linux has to work around it: https://github.com/archlinux/svntogit-community/blob/packages/python-poetry/trunk/poetry-completions-generator