Add `packaging.index`

[brettcannon]

I imagine it would provide types for the JSON index API and a way to parse HTML into JSON. Basically what https://github.com/brettcannon/mousebender/blob/main/mousebender/simple.py does.

[HarshithaJ28]

Hi @brettcannon - it looks like a genuinely useful addition. Is this still something you'd want a contribution on? If so, I'd love to take a first pass at it - starting with just typed models for the JSON index responses (mirroring mousebender's ProjectDetails shapes), and leaving the HTML-parsing side as a follow-up, unless you'd rather approach it differently. Happy to adjust based on whatever you think makes the most sense.

[brettcannon]

I haven't talked to @henryiii and @notatallshaw about it whether we want a new module for this or not. And I have a design in mind that's simpler than what's in Mousebender, so I wouldn't want to do it as-is.