预处理器指令在Package.swift中不起作用？

// swift-tools-version:5.5
// The swift-tools-version declares the minimum version of Swift required to build this package.

import PackageDescription

let package = Package(
    name: "MyPackage",
    platforms: [
        .macOS(.v10_15),
        .iOS(.v13),
    ],
    products: [
        .library(
            name: "MyLibrary",
            targets: ["MyLibrary"]),
    ],
    dependencies: [
        .package(url: "https://github.com/SomeDependency", from: "1.0.0"),
    ],
    targets: [
        .target(
            name: "MyLibrary",
            dependencies: ["SomeDependency"]),
        .testTarget(
            name: "MyLibraryTests",
            dependencies: ["MyLibrary"]),
    ]
)