[devctl] Optimize cleanup_sys_modules a bit by moving out of a loop a costly computation that is constant. This at least benefits the `i18ncube` command. Related to #15789486.

usr/lib/python2*/*-packages/cubicweb/entities/
usr/lib/python2*/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/python2*/*-packages/cubicweb/*.py