Do not generate public empty modules (#1803)

* Mark `operation_handler` module as private in the generated server code since it does not contain any public type.
Tune the visibility of the 'operation' module based on the rendering context

* Add changelog.next entry.

* Rename `operationModule` to `operation`.

* Ensure that `operation` is added to the list of public modules for the generated client.

* Remove DefaultModules from codegen-core

* Fix Python server.

* Expose TestDefaultPublicModules.