Change GTC repository layout

[egparedes]

Change the layout to have one top-level folder per sub-project.

Example:

REPO/
    gtc/
        src/
            gtc/
        docs/
        tests/
        setup.cfg
        setup.py
        pyproject.toml
        ...

    eve/
        src/
            eve/
        docs/
        tests/
        setup.cfg
        setup.py
        pyproject.toml
        ...

    gt_frontend/
        src/
        docs/
        tests/
        setup.cfg
        setup.py
        pyproject.toml
        ...

    cpputil/  # or gridtools-mesh or any other name
        cmake/
        include/
        tests/
        CMakeLists.txt
        ...