Add package path to sys when downloading package as github archive
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Sorry for the long diff, everything after the imports comes from `black` for code quality :/   I think it's fine and I can't think of another way to make the import work anyways.

Maybe we can have the `sys.path` behavior inside `prepare_module` instead ? Currently it seems to come out of nowhere in the code ^^'
We could check if external imports have a `__init__.py` and if it is the case then we can add to directory to the `PYTHONPATH`
This fixes the `coval.py` metric so that imports within the downloaded module work correctly. We can use a similar trick to add the BLEURT metric (@ankparikh)

@thomwolf not sure how you feel about adding to the `PYTHONPATH` from the script. This is the only way I could make it work with my understanding of `importlib` but there might be a more elegant method.

This PR fixes https://github.com/huggingface/nlp/issues/305
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