Support platform specific implementations

Package.swift

// swift-tools-version:4.0
// The swift-tools-version declares the minimum version of Swift required to build this package.

import PackageDescription

let package = Package(
    name: "CLI",
    products: [
        // Products define the executables and libraries produced by a package, and make them visible to other packages.
        .executable(
            name: "CLI",
            targets: ["CLI"]
        )
    ],
    dependencies: [
        // Dependencies declare other packages that this package depends on.
        // .package(url: /* package url */, from: "1.0.0"),
    ],
    targets: [
        // Targets are the basic building blocks of a package. A target can define a module or a test suite.
        // Targets can depend on other targets in this package, and on products in packages which this package depends on.
        .target(
            name: "CLI",
            dependencies: []
        ),
        .target(
            name: "UnzipLinux",
            dependencies: []
        ),
        .target(
            name: "UnzipMacOS",
            dependencies: []
        ),
        .target(
            name: "UnzipWin32",
            dependencies: []
        ),
    ]
)

#if os(Linux)

for target in package.targets {
    switch target.name {
    case "CLI":
        target.dependencies.append(.target(name: "UnzipLinux"))
    default: break
    }
}

#elseif os(OSX)

for target in package.targets {
    switch target.name {
    case "CLI":
        target.dependencies.append(.target(name: "UnzipMacOS"))
    default: break
    }
}

#elseif os(Windows)

for target in package.targets {
    switch target.name {
    case "CLI":
        target.dependencies.append(.target(name: "UnzipWin32"))
    default: break
    }
}

#endif