Gentle ping again :) I’ve also created a PR for this.

> The Debian/Ubuntu packages have a local patch for distutils/setuptools introducing an --install-layout option.  Maybe have the same for pip?

Pip already has a similar mechanism. The default layout is `prefix` and you can change the installation prefix with `--prefix`; `home` and `user` layouts can be specified with `--home={base}` and `--user`.

> So the problem to solve is
>
>  - let a "sudo pip install" fail by default on the real system
>  - let the same install succeed in a docker environment, or any other "image".

These need to be done in pip, so we’ll have a separate discussion on them elsewhere.

>  - behave transparently on venv and virtualenv installations.

This is what this issue tries to address. A distribution can overwrite `sysconfig.get_default_scheme()` and `sysconfig.get_preferred_scheme(variant)` to return the correct scheme based on whether the current Python is in a virtual environment (by detecting `sys.prefix` and `sys.base_prefix`, I think).

When in a virtual environment, it can return the same scheme as the upstream. Outside of a virtual environment, it can do whatever the platform sees fit, and pip (or whatever calls sysconfig) will install things into wherever it’s told to by the two functions.