Move `pip` to separate completion definition delegating to pip to do
all the completion work.
Since the completion is loaded lazily on-demand, it avoids the
performance overhead during Zsh initialization.
Implementation note:
The helper function `_pip_completion` implementation is based on the
official pip completion function (which can be generated with
`pip completion --zsh`) adhering to the newer compsys style.
See: 'man zshcompsys' for more details.
