Feature Request: Enhance recipe-generator to allow for local/private pypi index

[b-morgenthaler]

Currently, the URL to pypi.org/simple apparently is hard-coded:

rattler-build/src/recipe_generator/pypi.rs

Line 111 in c431478

PackageSources::from(url::Url::parse("https://pypi.org/simple/").unwrap());

It would be nice to allow for local/private pypi index as well - overwriting the URL by environment variable or by passing a CLI option.

This might bring some more details with it, like disable ssl verification (e.g. if local index is self-signed) or basic authentication with URL if local index is private.

A possible "workaround" for this use case currently: use Conda's Greyskull to generate meta.yml from local/private pypi index -> convert meta.yml into recipe.yaml by conda-recipe-manager -> build with rattler-build