Fix imports after recent upstream CloudFormation changes

[whummer]

Fix imports after recent upstream CloudFormation changes.

Resolving an issue recently reported by a user:

...
localstack-main  |   File "/var/lib/localstack/lib/extensions/python_venv/lib/python3.11/site-packages/aws_replicator/server/extension.py", line 14, in update_gateway_routes
localstack-main  |     from aws_replicator.server.request_handler import RequestHandler
localstack-main  |   File "/var/lib/localstack/lib/extensions/python_venv/lib/python3.11/site-packages/aws_replicator/server/request_handler.py", line 31, in <module>
localstack-main  |     from aws_replicator.server.resource_replicator import ResourceReplicatorFormer2
localstack-main  |   File "/var/lib/localstack/lib/extensions/python_venv/lib/python3.11/site-packages/aws_replicator/server/resource_replicator.py", line 12, in <module>
localstack-main  |     from aws_replicator.client.service_states import ExtendedResourceStateReplicator
localstack-main  |   File "/var/lib/localstack/lib/extensions/python_venv/lib/python3.11/site-packages/aws_replicator/client/service_states.py", line 6, in <module>
localstack-main  |     from localstack.services.cloudformation.models.s3 import S3Bucket
localstack-main  | ModuleNotFoundError: No module named 'localstack.services.cloudformation.models.s3'
localstack-main  | 2024-03-06T22:33:55.357  INFO --- [-functhread4] hypercorn.error            : Running on https://0.0.0.0:4566/ (CTRL + C to quit)

The imports are now deferred until actually required in the processing flow. In a future iteration, we need to fix the replicator "copy" logic entirely, it is currently mostly broken (only the replicator "proxy" logic is currently maintained).

/cc @lakkeger