Replace setup.py with pyproject.toml (#9021)

* Create pyproject.toml
* Implement a custom build backend (see below) in packager directory. Build logic from setup.py has been refactored and migrated into the new backend.
* Tested: pip wheel . (build wheel), python -m build --sdist . (source distribution)

tests/ci_build/lint_python.py

@@ -198,7 +198,7 @@ def main(args: argparse.Namespace) -> None:
             run_mypy(path)
             for path in [
                 # core
-                "python-package/xgboost/",
+                "python-package/",
                 # demo
                 "demo/json-model/json_parser.py",
                 "demo/guide-python/external_memory.py",