Use local schemas to define validation for pyproject.toml

[abravalheri]

Summary of changes

This facilitates adding/removing options without having to release a whole new version of validate-pyproject.

In general it should help to improve the developer experience when dealing with pyproject.toml validations.

Closes

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
  (See documentation for details)

[abravalheri]

@jaraco this is my preferred approach for dealing with the validations, the reason is that it allows contributors and maintainers of setuptools to modify the validations of pyproject.toml locally.

It also helps to clarify the relationships between validate-pyproject and setuptools: validate-pyproject "compiles" the JSON schemas that should be given by setuptools. This way "setuptools" is in charge and does not have to wait for changes in validate-pyproject to perform trivial validation changes.

[jaraco]

LGTM