I'm not sure about this method (loading these modules into ``sys.modules``).  This can lead to one subsubtest module clashing with a totally unrelated subsequent subtest module that happens to use the same subsubtest module name.  I'd really like to treat subtests (and dependencies) more like independent "plugins" and less like libraries.

My motivation in being overly-cautious here is because ldoktor already ran into a naaaasty problem caused by ``sys.modules`` pollution in the control file.  So if we can, I'd like  'subtests' (and dependent modules) to have **zero** chance to accidentally step on each other.

That said, what was the specific problem or situation this function is needed for?  Is there a different way we can solve that so we don't need to touch ``sys.modules``?  Maybe add a 'modules' class attribute to store these instead (all ``dockertest`` modules are unloaded between tests)?