Implement new JSON API Endpoints for new proposed PEP

[cooperlees]

What's the problem this feature will solve?
We're trying to make the JSON API documented in a PEP and more portable for alternate Package Index servers to implement a JSON API for each index.
e.g. devpi
This will allow:

• https://index.com/mirror_x/json
• https://index.com/mirror_y/json

Describe the solution you'd like
Add a new /json endpoint to pypi.org
As per PEP: https://github.com/cooperlees/peps/blob/warehouse_json_api/pep-9999.rst#api-urls
(will update URL as we progress)

Additional context
Draft PEP: https://github.com/cooperlees/peps/blob/warehouse_json_api/pep-9999.rst
PEP Issue: pypa/packaging-problems#367

[di]

@cooperlees Can this be closed now that we have PEP 691?

[cooperlees]

PEP691 is not really a 1:1 replacement. Users still use the JSON API for where PEP691 does not offer the metadata they need. So I still argue this is valid and a need to truly mirror all of PyPI in a way that allows mirrors to also have complete separate indexes.

We still don't have a standardized way to house different "JSON APIs" on a mirror via one mirror endpoint. We can say it's not a goal. I unfortunately don't ever see a compromise happening here. I've asked for so long that I've given up.

[di]

In that case do you think we can close this in favor of #12488? Or should this stay open to discuss support for "complete separate indexes" in addition to that?

[cooperlees]

I feel they ask for different but similar things. #12488 wants an easier API for getting changed since XML RPC calls basically and this wants the ability to make the API route/endpoint/path cleaner so multiple indexes can have their own respective "JSON API".

We need to decide if this should or will ever happen, I doubt it will, so we should probably just close and say we're going to focus on extending the Simple API like PEP 691 or make a new API. The /pypi/PKG_NAME/json route could also be left and new indexes just can't be called "pypi" which is probably a good idea anyways. Then mirroring could just use /$index_name/$pkg_name/json ...