def _package_top_dir(self):
        """
        Find one or more directories that we think may be the top-level
        directory of the package; return a list of their absolute paths.

        :return: list of possible package top-level directories (absolute paths)
        :rtype: list
        """
        r = [self.package_dir]
        for l in self._pip_locations:
            if l is not None:
                r.append(l)
        for l in self._pkg_resources_locations:
            if l is not None:
                r.append(l)
        return sorted(list(set(r)))