This property allows to simplify language compilation commands in multiple cases. The compilation commands presented to the user are also easier to understand.

The initial motivation for this change was to be able to use python3 -m zipapp instead of manual archiving for the Python 3 language. Currently this doesn't work, however, because from PEP 441 there must be a __main__.py file in the archive, and zipapp enforces this. But these changes are still useful for multiple other languages.

A couple of notes:

1. The .pyz extension is specified in PEP 441.
2. This code doesn't work if there are multiple source files, but no grader:
   

   cms/cms/grading/tasktypes/Batch.py

   Lines 256 to 257 in d4c9e92

   	main = self.GRADER_BASENAME \
   	if self._uses_grader() else executable_filename

   
   I think it could query the task object instead. Perhaps a new issue can be opened.
3. The PHP language plugin shows that it may be useful to have an InterpretedLanguage subclass without compilation commands. Currently a language with empty compilation commands fails here:
   

   cms/cms/grading/steps/utils.py

   Lines 88 to 89 in d4c9e92

   	stats = None
   	for exec_num, command in enumerate(commands):

   
   because stats is None. On the other hand, it is not clear how this can be reconciled with the case of multiple source files.
4. Logically, each language type plugin should implement its own updater function, so that the updater script can query all plugins, including out-of-tree ones. As this is overkill, the updater just has a hardcoded list of languages instead.

This change is