Fast start up
Currently if optional dependencies such as tensorflow, torch, apache_beam, faiss and elasticsearch are installed, then it takes a long time to do `import datasets` since it imports all of these heavy dependencies.

To make a fast start up for `datasets` I changed that so that they are not imported when `datasets` is being imported. On my side it changed the import time of `datasets` from 5sec to 0.5sec, which is enjoyable.

To be able to check if optional dependencies are available without importing them I'm using `importlib_metadata`, which is part of the standard lib in python>=3.8 and was backported. The difference with `importlib` is that it also enables to get the versions of the libraries without importing them.
I added this dependency in `setup.py`.