Move _validate_pyproject out of _vendor

[abravalheri]

_validate_pyproject is not a vendored version of validate_pyproject. Instead it consists of new code generated from the JSON Schema files.

To help OS re-packagers it would be best to move them out of _vendor.

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Context:

I think moving _validate_pyproject out of _vendor would also help us after all. Out unbundling logic is pretty much two regexps and while I suppose we could hack it around to exclude _validate_pyproject, it would be much easier if we didn't have to.

Originally posted by @mgorny in #2825 (comment)

[mgorny]

For the record, setuptools has some extern logic that's also affected by this, I think.