Ensure that SourceProcessor cache directories exist.

def create_cache_directories(self):
        """Ensure that SourceProcessor cache directories exist."""
        if not os.path.isdir(self.package_cache_dir):
            if not os.path.isdir(self.stacker_cache_dir):
                os.mkdir(self.stacker_cache_dir)
            os.mkdir(self.package_cache_dir)