[8.x] Guess relation name in model factories

[bastien-phi]

I think we could improve the way relationship are guessed.

Actually, the relationship is always pluralized and singular form of HasOne cannot be guessed. For example, a User having a has one subscription relation we always have to write
 

UserFactory::new()
    ->has(SubscriptionFactory::new(), 'subscription')
    ->create();

This PR resolves correctly the relation just doing

UserFactory::new()
    ->has(SubscriptionFactory::new())
    ->create();

I don't know if this should be used also on hasAttached method to be honest...

If this needs tests, I would be happy to add them.