Python: common method for overriding of libraries

[FRidh]

Issue description

Most Python library expressions are in python-packages.nix and some are called using callPackage.
We applied makeOverridable to buildPythonPackage so we can override the Python packages. However, callPackage = pkgs.newScope self; also sets a override attribute which is applied later thus taking precedence. We should provide a common method for overriding Python libraries.

Possible methods I can think of:

• Define callPackage without providing a override attribute. The override attribute should now correspond to the one added to buildPythonPackage by makeOverridable.
• Add a buildPythonPackage.overridePythonPackage method.

I suppose the first would be easiest and the second the most robust.

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It would by the way be nice if makeOverridable would allow you to set the attribute used for overriding.

[FRidh]

cc @copumpkin @lsix @Mic92 @abbradar

See also the implementation at #24155. Seems to work for me.

[FRidh]

This went in already a long time ago.