Add validate-pyproject as a vendored dependency

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 pypa#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).

pavement.py

@@ -1,8 +1,13 @@
+import os
 import re
 import sys
+import shutil
 import subprocess
+import venv
+import string
+from tempfile import TemporaryDirectory
 
-from paver.easy import task, path as Path
+from paver.easy import info, task, path as Path
 
 
 def remove_all(paths):
@@ -67,4 +72,40 @@ def update_pkg_resources():
 def update_setuptools():
     vendor = Path('setuptools/_vendor')
     install(vendor)
+    install_validate_pyproject(vendor)
     rewrite_packaging(vendor / 'packaging', 'setuptools.extern')
+
+
+def install_validate_pyproject(vendor):
+    """``validate-pyproject`` can be vendorized to remove all dependencies"""
+    req = next(
+        (x for x in (vendor / "vendored.txt").lines() if 'validate-pyproject' in x),
+        "validate-pyproject[all]"
+    )
+
+    pkg, _, _ = req.strip(string.whitespace + "#").partition("#")
+    pkg = pkg.strip()
+
+    opts = {}
+    if sys.version_info[:2] >= (3, 10):
+        opts["ignore_cleanup_errors"] = True
+
+    with TemporaryDirectory(**opts) as tmp:
+        venv.create(tmp, with_pip=True)
+        path = os.pathsep.join(Path(tmp).glob("*"))
+        venv_python = shutil.which("python", path=path)
+        info(f"Temporarily installing {pkg!r}...")
+        subprocess.check_call([venv_python, "-m", "pip", "install", pkg])
+        cmd = [
+            venv_python,
+            "-m",
+            "validate_pyproject.vendoring",
+            "--output-dir",
+            str(vendor / "_validate_pyproject"),
+            "--enable-plugins",
+            "setuptools",
+            "distutils",
+            "--very-verbose"
+        ]
+        subprocess.check_output(cmd)
+        info(f"{pkg!r} vendorized")