Move `METADATA.toml` to `.data`

Currently, `METADATA.toml` is added to each top-level package. This was done, since we had no better place at the time. In https://discuss.python.org/t/extra-files-in-dist-info/39418/4 it turns out that it's probably possible to add a `_typeshed` directory into `.dist-info`, which would be a better location for it. But even better would be the [`.data` directory](https://packaging.python.org/en/latest/specifications/binary-distribution-format/#the-data-directory). [hatchling supports this](https://hatch.pypa.io/latest/plugins/builder/wheel/#options) for wheels via the `shared-data` option. I'm not sure about setuptools or flit.