* Fix exception with "object has no attribute"
When use this code:
client = boto3.client('lambda')
client.get_policy([...])
moto rise:
```
moto/awslambda/responses.py", line 109, in _get_policy
lambda_backend = self.get_lambda_backend(full_url)
Exception: 'LambdaResponse' object has no attribute 'get_lambda_backend'
```
* fix shadows built-in name