In order to minimise dependencies, `validate-pyproject` has the ability
to "dump" only the code necessary to run the validations to a given
directory. This special strategy is used instead of the default
`pip install -t`.
The idea of using JSONSchema for validation was suggested in #2671,
and the rationale for that approach is further discussed in
https://github.com/abravalheri/validate-pyproject/blob/main/docs/faq.rst
Using a library such as `validate-pyproject` has the advantage of
incentive sing reuse and collaboration with other projects.
Currently `validate-pyproject` ships a JSONSchema for the proposed
use of `pyproject.toml` as means of configuration for setuptools.
In the future, if there is interest, setuptools could also ship its own
schema and just use the shared infrastructure of `validate-pyproject`
(by advertising the schemas via entry-points).