Xcode 11中没有这样的模块‘PackageDescription’

我有一个现有的项目框架和库，并希望他们使用SWIFT软件包管理器。

我有一个Xcode 11并且已经读过了用Xcode创建一个SWIFT包.

在添加Package.Swive之后

import PackageDescription

let package = Package(
    name: "MyLibrary",
    platforms: [
        .macOS(.v10_13),
    ],
    products: [
        .library(name: "MyLibrary", targets: ["MyLibrary"]),
    ],
    dependencies: [
        .package(url: "https://url/of/another/package/named/Utility", from: "1.0.0"),
    ],
    targets: [
        .target(name: "MyLibrary", dependencies: ["Utility"]),
        .testTarget(name: "MyLibraryTests", dependencies: ["MyLibrary"]),
    ]
)

我有个错误

没有这样的模块“PackageDescription”

如何导入PackageDescription模块？