Remove duplication of the labextension

[martinRenou]

Taking bqplot as a reference implementation for packaging the labextension. Thanks @maartenbreddels who implemented this approach.

This removes the duplication of the labextension in the wheel (which basically divises by two the size of the wheel for xeus-python-kernel, going from ~30MB to ~15MB).

@jtpio if you think this approach makes sense, I'm happy to upstream those changes into jupyterlab/extension-cookiecutter-ts and fix jupyterlab/extension-cookiecutter-ts#220

[martinRenou]

Actually we can get rid of the package.json under site-packages if we follow what bqplot does all the way through (finding the package.json in the prefix).

[maartenbreddels]

Thanks @maartenbreddels who implemented this approach.

Thanks :) What PR is this referring to btw?

[martinRenou]

It was for the nbextension in bqplot bqplot/bqplot#1228 and we followed together the same pattern for the labextension bqplot/bqplot#1222 :)