get(identifier, etag) {
		const info = this.itemInfo.get(identifier);
		this._addRequest(identifier);
		if (info === undefined) {
			return;
		}
		if (info.etag !== etag) return null;
		info.lastAccess = Date.now();
		const loc = info.location;
		if (loc === -1) {
			return info.freshValue;
		}
		if (!this.content[loc]) {
			return;
		}
		return /** @type {PackContent} */ (this.content[loc]).get(identifier);
	}
@param {string} identifier unique name for the resource
@param {string | null} etag etag of the resource
@returns {Data} cached content