…rtual environments sharing the same base.

The new functionality serves a use case which was not previously possible with pip configuration files, namely the situation where you have a base Python installation and want to influence the pip configuration for all derivative virtual environments *without* changing the config for all other environments on a machine (global), or for all other environment run by the same user (user). Concretely, this could be used for a centrally managed network mounted filesystem based Python installation, from which multiple users can build virtual environments and inside which a specific pip configuration is needed (e.g. an index URL).

Co-authored-by: Tzu-ping Chung <uranusjr@gmail.com>

…tation

This follows the discussion in https://github.com/pypa/pip/pull/11487/files#r988625394,
that despite the VIRTUAL_ENV environment variable not being the technically correct
value, it is more readily understood by readers than ``sys.prefix``.