Expose API to store PyPI JSON metadata into the conda-index cache

[danyeaw]

Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

Expose a method in index.py that accepts raw PyPI JSON, converts it to a v3 wheel record using translate.py, and stores it in the conda-index cache via cache.store().

Why is this needed?

External tools like conda-pypi-test should only need to fetch PyPI metadata and hand it off. Centralizing conversion and storage in conda-pypi means consumers become pure fetchers with no conversion logic of their own.

What should happen?

• Expose a method such as store_pypi_metadata(cache, pypi_json) that converts and stores a single package record
• Update update_index() in index.py to call ChannelIndex.index() with update_cache=False and update_only=True so repodata generation works without local files on disk

Additional Context

No response