Little cleanup for new purpose of the repo

[PicoCentauri]

No description provided.

[frostedoyster]

Explaining all the commits:

• first, we were relying on setup.py to build sdist and wheels, but there is no setup.py anymore
• second, we couldn't find the sdist to build the wheels from, because its name changed from metatensor-models to metatensor_models
• third, the final command of the build-python test environment only works if called from bash -c. No clue why.

I'm opening an issue to restore the naming to metatensor-models because that's more consistent with our other packages

[frostedoyster]

@PicoCentauri you can merge if you think this is ready. From here, I'll try to make some progress towards the MVP and you can clean up more