Dart fix + Flutter upgrade

lib/app_sources/apkpure.dart

@@ -8,7 +8,7 @@ import 'package:obtainium/providers/source_provider.dart';
 
 extension Unique<E, Id> on List<E> {
   List<E> unique([Id Function(E element)? id, bool inplace = true]) {
-    final ids = Set();
+    final ids = <dynamic>{};
     var list = inplace ? this : List<E>.from(this);
     list.retainWhere((x) => ids.add(id != null ? id(x) : x as Id));
     return list;
@@ -76,7 +76,7 @@ class APKPure extends AppSource {
     return Uri.parse(standardUrl).pathSegments.last;
   }
 
-  getDetailsForVersion(
+  Future<APKDetails> getDetailsForVersion(
     List<Map<String, dynamic>> versionVariants,
     List<String> supportedArchs,
     Map<String, dynamic> additionalSettings,