_getUnresolvedContextHash(path, callback) {
		const cache = this._contextHashes.get(path);
		if (cache !== undefined) return callback(null, cache);
		this.contextHashQueue.add(path, callback);
	}
@private
@param {string} path context path
@param {(err?: WebpackError | null, contextHash?: ContextHash | null) => void} callback callback function
@returns {void}