When a student script is rendered using render_one,
new packages might be added to the search path. These may conflict with the
instructors' search path order, or with subsequent runs of
render_one on students. Hence there is a need to reset the
search path before this is done.
This function does not unload namespaces. It only detaches them from the
search path. For a difference between the two, please see Hadley's page.