Allow download from alternative repositoires without authentication for pip via environment variables

[sinoroc]

In Poetry it is possible to specify alternative repositories for the download of dependencies...

pyproject.toml

[[tool.poetry.source]]
name = "foo"
url = "https://foo.bar/simple/"

Our plugin should be able to set the appropriate environment variables (PIP_INDEX_URL, PIP_EXTRA_INDEX_URL) so that pip can download distributions from those alternative repositories. To make things easier for a first step, we'll ignore authentication.

Related:

• https://python-poetry.org/docs/repositories/#install-dependencies-from-a-private-repository
• https://pip.pypa.io/en/stable/reference/pip_install/#cmdoption-index-url
• https://pip.pypa.io/en/stable/reference/pip_install/#cmdoption-extra-index-url
• https://pip.pypa.io/en/stable/user_guide/#environment-variables
• #8

[sinoroc]

I am not sure what we should do, in case the environment variables are already set outside of the tox.ini file, on the command line for example:

PIP_EXTRA_INDEX_URL=https://host.local/simple tox
PIP_INDEX_URL=https://host.local/simple tox