Move PYSPARK backend to the backends/ directory

Move the pyspark/ backend to backends/pyspark.

The current Ibis directory structure is a bit confusing, since backends are mixed with core modules. For example, it’s not obvious to know if pandas/ is a backend, or core functionality. Many of the backends live in the root directory, and some are grouped, like in a directory sql/. This makes things even harder and difficult to maintain. Also, ibis uses attributes to access the backends (e.g. ibis.spark.connect), and ibis.spark in code seems to be the ibis/spark/ module, but it’s not, making things confusing, and even creating problems with pytest.

We’re moving backends to a standardized structure, in the backends/ directory, one backend at a time (each backend in a separate pull request). We started by the OmniSciDB backend, and its PR can be a reference for the changes that need to be made here: #2392

The exact tasks to perform are:

• Move the backend from its original location to backends/ (if the backend is in a subdirectory, like sql/, ignore the subdirectory and just have the backend inside backends/, so sql/sqlite/ -> backends/sqlite)
• Move the content of backends/<your-backend>/api.py to backends/<your-backend>/__init__.py. Backends are using api.py for historical reasons, but the public backend API should be in the standard __init__.py file of the module.
• Update ibis/__init__.py so the backend is imported from the new module (from ibis.<your-backend> import api -> from ibis import <your-backend>)
• Look for places in the code where your backend is being imported, and update the imports. For imports inside the same backend, please use relative imports (e.g. import ibis.<your-backend>.whatever -> import .whatever). See the reference PR for examples.
• Update the documentation (in doc/source/backends/<your-backend>.rst) so the API is generated from the right location.

After performing the changes, the CI should be green, except for the packaging build (which requires changes in the conda-forge feedstock). You can ignore that error, those changes will be performed later for all moved backends.

[datapythonista]

No, this was closed by #2479.

All the moving has been done now, the only thing that is pending are couple of files left of #2466. If you want to move that, it would be great.

Something else that would be very valuable, and I think it shouldn’t be so complex is #2389. There is a PR open, but the approach ended up not being great. So working on the original approach of making the scripts work on windows would be better. And the CI has been giving us lots of trouble lately, so being able to move everything to Actions would be really great.

[datapythonista]

The content of api.py needs to be moved to __init__.py, and api.py needs to be removed.