Isolate imports and conditionally import libraries to make package platform-independent

## 🏗 Enhancement Proposal

Currently `flutter_cache_manager` is platform-independent, yet according to pub.dev, the package doesn't support web. This can be solved using conditionally importing `dart:io` dependent libraries.
`path_provider`, `file` and `sqflite` are platform-dependent and must only be imported/exported in *_io.dart files which will then be conditionally imported/exported.

`file` can be replaced with `cross_file` (as mentioned in [#321 (comment)](https://github.com/Baseflow/flutter_cache_manager/pull/321#issuecomment-854064553))

`path_provider` and `sqflite` are rather easy to isolate since they are imported only in cache_info_repositories which are selectively imported in config_*.dart files. The only problem is that `cache_info_repositories/cache_info_repositories.dart` is exported. @renefloor what can we do about this?

### Pitch

This will increase pub points which affect discoverability. Also, partially platform-dependent code will be separated and be easier to maintain.

### Checklist

- [ ] #339
- [ ] Store the config json data as a file on all platforms except the web, where web storage will be used to store this data
- [ ] Make the JSON implementation of cache info repository platform independent
- [ ] Migrate all instances to use the new JSON implementation
- [ ] If necessary, replace platform dependent dependencies with cross-platform implementations

Commencez par suivre les imports et exports de path_provider, file et sqflite dans cache_info_repositories/cache_info_repositories.dart et les fichiers config_*.dart. Consultez la discussion liée #321 et la checklist, puis déterminez comment les imports conditionnels et le stockage multiplateforme des informations de cache devraient couvrir les plateformes web et non web. Le travail est considéré comme terminé lorsque les bibliothèques dépendantes de la plateforme sont isolées et que le package prend en charge web conformément à l’objectif indiqué.

Rédigé par le modèle d'indexation à partir du texte de l'issue.