feat: PEP 794 support (METADATA 2.5)

[henryiii]

• Still needs to verify no names match between the two lists. Maybe verify that these are valid Python identifiers (not keywords), dot separated with an optional ; private.
• Tests
• I'm not differentiating no entry vs. empty entry, but it looks like in the PEP that does write slightly different metadata.

(Inspired by pypa/packaging#948)

[henryiii]

I've added these two checks:

• Verify no names gets listed twice (combining both fields)
• Verify that 'spam.bacon.eggs' has 'spam' and 'spam.bacon' listed somewhere too. This is described in the PEP, but there's no all-caps SHOULD or similar for it.

I didn't add this one:

• I think any package that is listed in import-names should not have any children listed. If you have a 'spam' in import-names, then you don't need to list 'spam.bacon.eggs'? Not sure on that one, haven't implemented it.

@brettcannon, thoughts, does that sound right?

[brettcannon]

• Verify that 'spam.bacon.eggs' has 'spam' and 'spam.bacon' listed somewhere too. This is described in the PEP, but there's no all-caps SHOULD or similar for it.

That's a typo on my part; pypa/packaging.python.org#1939 .

• I think any package that is listed in import-names should not have any children listed. If you have a 'spam' in import-names, then you don't need to list 'spam.bacon.eggs'?

That's covered by, "Projects SHOULD list all the shortest import names that are exclusively provided by the project." So if you want to be strict then your proposal makes sense it just isn't technically illegal as long as there isn't some weird namespace interleaving (e.g. a and a.b.c are in import-names but import-namespaces has a.b).

[henryiii]

I'm combining names and namespaces before doing checks, so I could add it (also in abravalheri/validate-pyproject#271), then we could always be less strict in the future if someone needs it.

Ah, no, not for this check, I see. Okay, yes, I think you could have a normal package with a namespace package inside of it. I'll leave off such a check.

[henryiii]

Okay, so I don't think the extra check is needed. I'll merge this too then if abravalheri/validate-pyproject#271 passes @abravalheri's review.