fix package conflict by moving localstack dep to dev extra

[alexrashed]

Motivation

With localstack/localstack#11190, the localstack module is now an implicit namespace module.
However, in the latest release (3.5) this was not yet the case.
This uncovered an issue with the extensions if they define localstack-core (either directly or transitively via localstack-core) as an install-dependency:

• When installing the extension, the python package install dependencies are also installed into the extension virtual environment.
• When the extension defines an install-dependency on localstack-core, it will install the latest release of localstack-core into the extension virtual environment.
  • The extension virtual environment is directly linked to the main virtual environment in the LocalStack container and should directly use the code from the image (and not install it as an additional external dependency).
  • However, this currently causes a conflict because the installed version 3.5 - since it's not an implicit namespace package yet - which overwrites the module in the main venv.

[alexrashed]

@HarshCasper @whummer: The CI for the miniflare extension is failing, but it seems to me as this is not related to the namespace package or this fix.

[dominikschubert]

LGTM. Thanks for addressing these, should now avoid the clash between the localstack packages in the python path 🥳