Migrate `pip show`'s `Requires` field to `Requires-Dist`

[xavfernandez]

Prefer Requires-Dist to Requires to match PEP-0354 metadata.

This would require a transition period to not break user expectation: we might want to expose the same information in both Requires & Requires-Dist for a few pip version.

Originally posted by @sbidoul in #7967

[sbidoul]

Note my remark was more in the context of json exports than the current pip show output.

[xavfernandez]

Note my remark was more in the context of json exports than the current pip show output.

Sorry I misinterpreted your comment.

I think the "default/legacy" output & "json" one should share the same headers to ease switching from one to the other, meaning that if we want to use Requires-Dist, we'd want to use it in both our format.

[sbidoul]

Also do we want to harmonize json output across commands? pip list has a json option with lowercase keys, for instance.

[uranusjr]

Lower-casing sounds like a good idea. Metadata keys are case insensitive (RFC 5322), so it makes sense to me that the JSON equivalent has a clear case canonicalisation rule.

[xavfernandez]

Lower-case is also fine for me in json output.

But if we put requires-dist as a key in the json output, it would make sense to also have it spelled Requires-Dist in the "default" output (and not simply Requires).

[sbidoul]

Linking to #8008 for the discussion on the json format.