Implement PEP 794 (Import Name Metadata)

[brettcannon]

Closes #945

[henryiii]

In working on pypa/pyproject-metadata#260, I've thought of a few extra checks that might make sense there, or in both places:

• 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 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.

[henryiii]

I think this should test the empty Import-Name case - that's a very special case. It's the only place where an empty field is written; I know because I had to update the writer for pyproject-metadata to differentiate None vs. empty string to get this to write out there. Not sure where to test it though, everything.metadata just checks the ability of the email module to read it, it's not testing Metadata for example. I'm almost sure the as_rfc822 method will not write this correctly, it's a special case there too (writing an empty list as one empty element).

[henryiii]

I've got a patch ready that adds testing and fixes both directions of the special case. Would you like me to push it directly or make a PR?

[brettcannon]

I've got a patch ready that adds testing and fixes both directions of the special case. Would you like me to push it directly or make a PR?

Wonderful! Just push it to the PR.

[brettcannon]

Thanks for the improvements, @henryiii !