[5.0] Make use of new resolvingType method

Follow-up PR of #6724

Includes

- More consistent `short_array_syntax` in `Container` class
- More method documentation for new methods
- Methods now also added to the `Container` contract
- Made use of the new `$container->resolvingType` method to clean up `ValidationServiceProvider` and `FormRequestServiceProvider`.
They're unused imports even. Forgot to remove them again.