Option to import `pyi` stubs as modules

[LecrisUT]

The usecase for this is when we are building the documentation and using autodoc to avoid import failures and avoid cmake building. The user could have something like this setup

[[tool.scikit-build.overrides]]
if.any.env.READTHEDOCS = true
if.any.env.BUILD_DOCS = true
wheel.cmake = false

The logic to be added is that if there is a .pyi stub file without a .py original, then:

• regular: copy the stub file to a .py file
• edittable installs: probably ScikitBuildRedirectingFinder needs to be changed to detect .pyi files as modules