ENH Support alternative index urls

[ryanking13]

Resolve #7

This PR allows users to specify an index URL from which micropip can download packages. This allows users to install packages from a custom package index that provides PyPI-compatible APIs.

This PR covers both methods: modifying the index URL globally (micropip.set_index_urls) and passing the index URL as a parameter to micropip.install. This allows service operators like pyscript / jupyterlite to add their own package indexes as a default setting, while also allowing users to choose and use multiple indexes when needed.

• changelog

[rth]

Very nice! A few comments otherwise LGTM. Thanks @ryanking13 !

[rth]

Should it be search_package if it searches just 1 package? Also, it might be more user friendly to name the first argument name.

It's not "search" technically though. Search is returning top k results for a query, possibly with some pre-processing or fuzzing of the query. While this return a single response with which is the exact match. Maybe query_package? But I'm open to other name suggestions.

[ryanking13]

Thanks for the suggestion! I wasn't aware of the difference between search and query :) I will go with query_package.

[ryanking13]

I removed the <package_name> placeholder limitation so that index URLs for Simple APIs (e.g. https://pypi.org/simple) can be passed without modification.