Add conda-auth support for private PyPI index

[danyeaw]

What is the idea?

Enable conda-pypi to authenticate with private PyPI repositories by integrating with the conda-auth plugin. This would extend the --index functionality from issue #70 to support repositories that require authentication.

Why is this needed?

Many organizations use private PyPI repositories with various authentication providers. Currently, conda-pypi can only access public indices. Also #77 adds anacona-auth support, but that is only for users of anaconda itself.

What should happen?

Users should be able to authenticate with private PyPI indices using the same credential management system as conda channels. The workflow would be:

conda auth login --token <token> test.pypi.org
conda pypi install --index https://test.pypi.org/simple cowsay
Using custom PyPI index: https://test.pypi.org/simple
Authentication via conda-auth is available
Converting PyPI packages to conda format: \ Channels:
 - file:///Users/dyeaw/Library/Application%20Support/pypi
 - defaults
Platform: osx-arm64
Collecting package metadata (repodata.json): done
Solving environment: done
done
Installing converted packages: \ Channels:
 - file:///Users/dyeaw/Library/Application%20Support/pypi
 - defaults
Platform: osx-arm64
Collecting package metadata (repodata.json): done
Solving environment: done
\
## Package Plan ##

  environment location: /Users/dyeaw/miniconda3/envs/my-env

  added / updated specs:
    - cowsay


The following packages will be downloaded:

    package                    |            build
    ---------------------------|-----------------
    cowsay-6.1.2               |           pypi_0          24 KB  file:///Users/dyeaw/Library/Application%20Support/pypi
    ------------------------------------------------------------
                                           Total:          24 KB

The following NEW packages will be INSTALLED:

  cowsay             pypi/noarch::cowsay-6.1.2-pypi_0



Downloading and Extracting Packages:

Preparing transaction: done
Verifying transaction: done
Executing transaction: done
done

Additional Context

This builds on the --index support from issue #70 and leverages the existing conda-auth plugin infrastructure. When implementing this with #77, both plugins can serve as auth backends that can be selected based on which ones are installed and have tokens for the domain.

[danyeaw]

On the experimental https://github.com/danyeaw/conda-pypi/tree/pupa-merge branch there is a working version of this.