Rename provides_extras to provides_extra

[charliermarsh]

Summary

This is now consistent with requires_dist (singular).

[konstin]

Should we rename metadata.provides_extra too?

[charliermarsh]

@konstin -- Do you mind skimming again? I had to change this on a bunch of other structs once I followed the thread from your comment.

[konstin]

Thanks, I renamed the tiny remaining items.

[konstin]

I think the PR is ready to go as-is.

Since we're already bumping the cache version, we should decide before the next release whether we also want to change package.metadata.provides-extras in uv.lock to package.metadata.provides-extra (minor version bump) and provides-extras in tool.uv.dependency-metadata to provides-extra. The latter would happen without dropping support for the provides-extras alias, we only need to update the docs to recommend the singular term. I'd do those in follow-up PRs for size and release notes.

[charliermarsh]

I think I did change tool.uv.dependency-metadata to use provides-extra so let me just update the docs there (without removing the alias).