[fix] don't mix loaded modules with imported modules (sys.modules)

The utils.load_module() function is used to load a python file (aka module) and
return the module's namespace.  SearXNG uses this function to load *engines and
answerers* from arbitrary locations with arbitrary modifications.  These are not
real python modules and it is not intended to mix this *engines and answerers*
with the python modules registered in sys.modules.

Closes: https://github.com/searxng/searxng/issues/312
Suggested-by: @dalf in https://github.com/searxng/searxng/issues/312


Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>