Remove some horrifying plugin-finding code
Remove some horrifying plugin-finding code

Now we have an explicit list of PLUGINS to load for the autograder, instead of
blindly loading all the files in the autograder directory. This fixes a number
of problems caused by supporting files being loaded as autograder plugins (e.g.
binary.py, hmmmAssembler.py, etc.)

CSGreater-Developers/HMC-Grader,CSGreater-Developers/HMC-Grader,CSGreater-Developers/HMC-Grader,CSGreater-Developers/HMC-Grader,CSGreater-Developers/HMC-Grader