Make dwave-optimization a native namespace package

We'd like to change `dwave-optimization` to a native namespace package by removing `dwave/__init__.py`. However, doing so results in Cython builds failing. This is because Meson ultimately uses the `cython ...` command and that command cannot handle native namespace packages with relative imports, see https://github.com/cython/cython/issues/5335, https://github.com/cython/cython/issues/6287.

For now we leave a placeholder `dwave/__init__.py` in place, then discard it from the built wheel, see https://github.com/dwavesystems/dwave-optimization/pull/305.

I'll look into opening a issue/PR on Cython, as the linked issues are slightly different.