[8.x] Change __call factory and relation resolution

[bastien-phi]

This PR aims to resolve 2 problems we could have using magic forXX and hasXX of Illuminate\Database\Eloquent\Factories\Factory

1 : Inconsistency of parameter given to factoryForModel
As described in laravel/ideas#2284, if App\User uses Illuminate\Database\Eloquent\Factories\HasFactory, calling App\User::factory() will call Factory::factoryForModel with 'App\User' whereas calling PostFactory::forUser will call it with 'user'. IMHO it should always be called with the fully qualified class name.

2 : Relations are not always called by related model name
It is common to have relation named by something else than the related model name. For example, if a post belongs to a user, the post relation can be named author.
This PR allows developer to call $postFactory()->forAuthor() to create the post's author.

[bastien-phi]

Thanks !