Use bazel's downloader for downloading pip packages

[cameron-martin]

🚀 feature request

Relevant Rules

The relevant rule is whl_library.

Description

Currently whl_library downloads pip packages by executing a python script. However this bypasses bazel's downloader, causing us to lose a few nice features such as:

• The ability to rewrite these urls to a mirror by using --experimental_downloader_config.
• Sharing the downloader cache between machines to speed up downloads.

Describe the solution you'd like

It would be nice to use bazel's downloader for these, by using repository_ctx.download.

Describe alternatives you've considered

Not doing this and keeping it like it is, I suppose.

[github-actions]

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