fix(workmanager_apple): Correct SPM structure and product name for Flutter compatibility

- Move Package.swift to ios/workmanager_apple/ subdirectory (Flutter SPM requirement)
- Rename product from 'workmanager_apple' to 'workmanager-apple' (Flutter naming convention)
- Add missing UIKit import in Extensions.swift for UIBackgroundFetchResult
- Move Sources, Resources, Assets to correct subdirectory

This fixes Swift Package Manager integration with Flutter's SPM plugin system,
which expects:
1. Package.swift at ios/<plugin_name>/Package.swift
2. Product names with hyphens (e.g., 'workmanager-apple' not 'workmanager_apple')

Without these fixes, Flutter falls back to CocoaPods instead of using SPM.

Author: Iwan Gerber

workmanager_apple/ios/Assets/.gitkeep renamed to workmanager_apple/ios/workmanager_apple/Assets/.gitkeep

@@ -10,7 +10,7 @@ let package = Package(
     ],
     products: [
         .library(
-            name: "workmanager_apple",
+            name: "workmanager-apple",
             targets: ["workmanager_apple"]
         )
     ],

workmanager_apple/ios/Package.swift renamed to workmanager_apple/ios/workmanager_apple/Package.swift

@@ -6,6 +6,7 @@
 //
 
 import Foundation
+import UIKit
 
 extension UIBackgroundFetchResult: CustomDebugStringConvertible {
     public var debugDescription: String {