Typing aliases are missing

Type aliases are currently missing in places, leading to crazy return signatures such as:

```python
Union[
    Callable[
        [Callable[Concatenate[Dict[str, Any], _P], Dict[str, Any]]],
        AwsEventRawCallable,
    ],
    Callable[
        [Callable[Concatenate[AwsEventType, _P], LambdaResponse]],
        AwsEventRawCallable,
    ],
]
```