A package set is constructed for a specific interpreter. Therefore, we add the possibility to override the package set to the interpreter. This should make it easier to override the interpreter and the package set at the same time.
Fixes https://github.com/NixOS/nixpkgs/issues/18484
This reverts commit 6b11c96c86.
6b11c96c86
Move Python interpreters (CPython, PyPy) to same folder and share layout.