Hint at missing `project.name` (#6803)
We got user reports where users were confused about why they can't use
`[project.urls]` in `pyproject.toml` (i think that's from poetry?). This
PR adds a hint that (according to PEP 621), you need to set
`project.name` when using any `project` fields. (PEP 621 also requires
`project.version` xor `dynamic = ["version"]`, but we check that later.)
The intermediate parsing layer to tell apart syntax errors from schema
errors doesn't incur a performance penalty according to epage
(https://github.com/toml-rs/toml/issues/778#issuecomment-2310369253).
Closes #6419
Closes #6760
