def referrers(self) -> set[str]:
        """A set of entries in referrers file"""
        if self._referrers is None:
            filepath = self.path / ".referrers"
            if not filepath.is_file():
                return set()
            self._referrers = {
                line.strip()
                for line in filepath.read_text("utf8").splitlines()
                if line.strip() and os.path.exists(line.strip())
            }
        return self._referrers
A set of entries in referrers file
src/pdm/models/cached_package.py