Add system framework search path on Package.swift

Package.swift

// swift-tools-version: 5.8
// The swift-tools-version declares the minimum version of Swift required to build this package.
import Foundation
import PackageDescription

let isXcodeEnv = ProcessInfo.processInfo.environment["__CFBundleIdentifier"] == "com.apple.dt.Xcode"
// Xcode use clang as linker which supports "-iframework" while SwiftPM use swiftc as linker which supports "-Fsystem"
let systemFrameworkSearchFlag = isXcodeEnv ? "-iframework" : "-Fsystem"

let package = Package(
    ...
    targets: [
        .target(
            name: "Sharing",
            linkerSettings: [
                .unsafeFlags([systemFrameworkSearchFlag, "/System/Library/PrivateFrameworks/"]),
            ]
        ),
    ]
)

swiftlang/swift-package-manager#6512