We got user reports where users were confused about why they can't use `[project.urls]` without other `pyproject.toml`. 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 (toml-rs/toml#778 (comment)).

Closes #6419
Closes #6760