use zsh completion cache for costly python module completions

zsh completion is very slow when completing on Python modules:

`python -m <TAB>` or `python -m <partial_match><TAB>` invokes
```python
from pydoc import help; help("modules")
```
on every `<tab>` which is quite slow and prone to spewing messages to stderr. Using the zsh completion cache functions speeds it up immensely.