Decouple shiboken and shibokengenerator

Currently the blocker is with `shibokenmodule` depending on `Shiboken6::shiboken6` to generate:

• `shiboken_module_wrapper.cpp`
• `shiboken_python.h

Which in turn creates the `Shiboken` python module file. But it seems I got the dependency backwards. Neither generator nor the apiextractor actually depend on shiboken libraries. So instead, we should look at packaging shiboken_generator as a standalone and build shiboken library with it. I.e.

[build-system]
requires = ["scikit-build-core", "shiboken6_generator"]
build-backend = "scikit_build_core.build"

[project]
name = "shiboken6"
dependencies = [  ]

[build-system]
requires = ["scikit-build-core"]
build-backend = "scikit_build_core.build"

[project]
name = "shiboken6_generator"
dependencies = [ "shiboken6" ]

(not sure if all shiboken6_generator dependency should be moved to shiboken6)

Another uncertainty is if the buildsystem can be convinced to not check for the `dependencies` during the build process, which I think it would be the case.