Avoid pip install --target

See https://github.com/pypa/pip/issues/4390 .

IMO the pip rules should create a venv , and `--root`. Ideally even they have a optional venv name argument, so that the same venv may be re-used across several rules , allowing PyOxidizer to complete pre-reqs first, effectively allowing multiple phases of requirements to workaround the many problems of pip.

It also would mean that the pip rules could do some pip prep , and then the re-use the VirtualEnv logic.

Start by tracing the pip rules and the existing VirtualEnv logic. Compare the current `pip install --target` approach with the proposed venv and `--root` flow, including optional venv reuse across rules and multiple requirement phases. Done means the rules can prepare and reuse a virtual environment without the problems described in pip issue 4390.

Written by the indexing model from the issue text.