Fixed 6.6 nullref caused by getting the removed GetPackedSprites method

[TNQKhang]

Unity removed the private SpriteAtlasExtensions.GetPackedSprites method in Unity 6.6, so opening the asset detector window generates this nullref, rendering it unusable:

ArgumentNullException: Value cannot be null.
Parameter name: method
System.Delegate.CreateDelegate (System.Type type, System.Object firstArgument, System.Reflection.MethodInfo method, System.Boolean throwOnBindFailure, System.Boolean allowClosed) (at <c5ae8a4eeb824526b5df2932bc2395e0>:0)
System.Delegate.CreateDelegate (System.Type type, System.Reflection.MethodInfo method, System.Boolean throwOnBindFailure) (at <c5ae8a4eeb824526b5df2932bc2395e0>:0)
System.Delegate.CreateDelegate (System.Type type, System.Reflection.MethodInfo method) (at <c5ae8a4eeb824526b5df2932bc2395e0>:0)
AssetUsageDetectorNamespace.AssetUsageDetector..cctor () (at ./Library/PackageCache/com.yasirkula.assetusagedetector@69f7ee5a27fc/Plugins/AssetUsageDetector/Editor/AssetUsageDetectorSearchFunctions.cs:187)
Rethrow as TypeInitializationException: The type initializer for 'AssetUsageDetectorNamespace.AssetUsageDetector' threw an exception.
AssetUsageDetectorNamespace.AssetUsageDetectorWindow..ctor () (at ./Library/PackageCache/com.yasirkula.assetusagedetector@69f7ee5a27fc/Plugins/AssetUsageDetector/Editor/AssetUsageDetectorWindow.cs:49)
UnityEngine.ScriptableObject:CreateInstance()
AssetUsageDetectorNamespace.AssetUsageDetectorWindow:GetWindow(WindowFilter) (at ./Library/PackageCache/com.yasirkula.assetusagedetector@69f7ee5a27fc/Plugins/AssetUsageDetector/Editor/AssetUsageDetectorWindow.cs:169)
AssetUsageDetectorNamespace.AssetUsageDetectorWindow:OpenActiveWindow() (at ./Library/PackageCache/com.yasirkula.assetusagedetector@69f7ee5a27fc/Plugins/AssetUsageDetector/Editor/AssetUsageDetectorWindow.cs:188)

I added new GetPackedSprites logic similar to the one in SpriteAtlasReportUtility in the Unity 2D Tooling package.
Differences between the two new and old behaviours:

• Old behaviour: doesn't count sub-assets if Sprite Atlas packing is set to Enabled for builds or Disabled.
• New behaviour: always counts the sub-assets, even if Sprite Atlas packing is set to Disabled.