feat!: 450 - import only "openfoodfacts.dart"; all code in "src" fold…

…er (#655)

New file:
* `open_food_api_client.dart`: contains the _code_ previously stored in `openfoodfacts.dart`

Impacted files:
* `openfoodfacts.dart`: now only `export`ing all relevant files
* test files: now they import only `openfoodfacts/openfoodfacts.dart`
* `product_state.dart`: renamed from `state.dart` to `ProductState`, as `State` is used for `StatefulWidget`s and that would make upgrade painful
* `nutrient_levels.dart`: renamed `Level` as `NutrientLevel` to fix confusion with `Level` in `knowledge_panel.dart`
* all code files: moved to `src` folder; now `import`ing relative files instead of openfoodfacts package files

example/lib/main.dart

@@ -1,9 +1,6 @@
 import 'dart:async';
 
-import 'package:openfoodfacts/model/ocr_ingredients_result.dart';
-import 'package:openfoodfacts/model/product_result_v3.dart';
 import 'package:openfoodfacts/openfoodfacts.dart';
-import 'package:openfoodfacts/utils/tag_type.dart';
 
 /// request a product from the OpenFoodFacts database
 Future<Product?> getProduct() async {