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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def unresolve_filename(self, package_dir, filename):
 """Retrieves the probable source path from the output filename. Pass in a .css path to get out a .scss path. :param package_dir: the path of the package directory :type package_dir: :class:`str` :param filename: the css filename :type filename: :class:`str` :returns: the scss filename :rtype: :class:`str` """

        filename, _ = os.path.splitext(filename)
        if self.strip_extension:
            for ext in ('.scss', '.sass'):
                test_path = os.path.join(
                    package_dir, self.sass_path, filename + ext,
                )
                if os.path.exists(test_path):
                    return filename + ext
            else:  # file not found, let it error with `.scss` extension
                return filename + '.scss'
        else:
            return filename