build system uses leftover .so files #4643
Description
As it stands, if you remove a Cython extension from module_list.py, and remove all associated files in the sage library, everything builds fine. However, the .so files are still there. In particular, if you try to load a pickled object from a class that was defined in that .pyx file, it still loads just fine -- in fact, it loads the .so and uses that code.
Unfortunately, I don't see an easy fix for this offhand. The problem is that we don't manage the .so files ourselves -- we leave that to distutils. If someone has a good idea for how to fix this, I'm happy to help implement it.
CC: @robertwb
Component: build
Reviewer: Jeroen Demeyer
Issue created by migration from https://trac.sagemath.org/ticket/4643