Find distribution packages.

def distribution_files(self) -> Iterator[str]:
        """Find distribution packages."""
        # This is verbatim from flake8
        if self.distribution.packages:
            package_dirs = self.distribution.package_dir or {}
            for package in self.distribution.packages:
                pkg_dir = package
                if package in package_dirs:
                    pkg_dir = package_dirs[package]
                elif '' in package_dirs:
                    pkg_dir = package_dirs[''] + os.path.sep + pkg_dir
                yield pkg_dir.replace('.', os.path.sep)

        if self.distribution.py_modules:
            for filename in self.distribution.py_modules:
                yield "%s.py" % filename
        # Don't miss the setup.py file itself
        yield "setup.py"