buildPython*: Deprecate and remove (buildPython* { ... }).override (#333670)

- The nvidia driver no longer defaults to the proprietary driver starting with version 560. You will need to manually set `hardware.nvidia.open` to select the proprietary or open driver.

- The `(buildPythonPackage { ... }).override` attribute is now deprecated and removed in favour of `overridePythonAttrs`.

  This change does not affect the override interface of most Python packages, as [`<pkg>.override`](https://nixos.org/manual/nixpkgs/unstable/#sec-pkg-override) provided by `callPackage` shadows such a locally-defined `override` attribute.

- All Cinnamon and XApp packages have been moved to top-level (i.e., `cinnamon.nemo` is now `nemo`).

- All GNOME packages have been moved to top-level (i.e., `gnome.nautilus` is now `nautilus`).

  else

    ./python2/mk-python-derivation.nix;

  buildPythonPackage = makeOverridablePythonPackage (lib.makeOverridable (callPackage mkPythonDerivation {

  buildPythonPackage = makeOverridablePythonPackage (callPackage mkPythonDerivation {

    inherit namePrefix;     # We want Python libraries to be named like e.g. "python3.6-${name}"

    inherit toPythonModule; # Libraries provide modules

  }));

  });

  buildPythonApplication = makeOverridablePythonPackage (lib.makeOverridable (callPackage mkPythonDerivation {

  buildPythonApplication = makeOverridablePythonPackage (callPackage mkPythonDerivation {

    namePrefix = "";        # Python applications should not have any prefix

    toPythonModule = x: x;  # Application does not provide modules.

  }));

  });

  # Check whether a derivation provides a Python module.

  hasPythonModule = drv: drv?pythonModule && drv.pythonModule == python;