Switch internal resource loading to importlib-resources

[omry]

importlb-resources is the next gen Python resource loader and will become the standard one in Python 3.9 (importlib.resources).
It has several advantages over pkg_resources, such as better performance and more consistent and well designed API.

pkg_resources has some side effects related to pytest plugins that are also a reason to avoid it.

Hydra 1.0 is switching to importlib-resources based ConfigSource for loading resources from packages, and the current pkg_resources implementation will be replaced.
for simplicity there will be no phase when both approaches are supported at the same time.
For the most part, this should be transparent to users.

Hydra will be more strict about requiring an __init__.py in config modules moving forward when the included config is loaded from the module and not the file system and in general it is now recommended to always include an __init__.py file at the top config modules or directories.

NOTE:
People using Hydra from the master will have to reinstall hydra-core to run the tests and may have to uninstall hydra-pytest-plugin (if they have it installed).