Impacted files:
* `paged_to_be_completed_product_query.dart`: renamed `State` as `ProductState`
* `paged_user_product_query.dart`: created local `UserSearchType` instead of deprecated off-dart `UserProductSearchType`; used `ProductSearchQueryConfiguration` instead of deprecated `UserProductSearchQueryConfiguration`
* `product_query.dart`: now that we use fully v3, we don't need the temporary field list for user-related queries with v2
* `pubspec.lock`: wtf
* `pubspec.yaml`: upgrade to off-dart 2.0.0
* `user_preferences_account.dart`: now using new local class `UserSearchType`
* and reduced all off-dart imports to a single `import 'package:openfoodfacts/openfoodfacts.dart';`
