[WIP] Introduce a new editable mode `build-dir`

[LecrisUT]

The goal of this new mode is to keep all the artifacts in either the source-dir or the CMake build-dir. From there we reuse the _editable_redirect.py as having only known_source_files and we include the build-dir files in that mapping.

In order to avoid conflicts with changes in #808, we should limit the scope here to python loadable sources and not cover importlib.resource.files right now. I will try to still gather those, but will filter those out of the _editable file for now until we figure out how we should pass them.

TODO:

• [ ] Get the source files mapping for the build-dir
• [ ] Test coverage

Closes #1160