Add testing for running as module with HYDRA_MAIN_MODULE

[omry]

different cases:

1:

main.py
config.yaml

2:

main.py
conf/config.yaml

3:

module/main.py
module/config.yaml

4:

module/main.py
module/conf/config.yaml

Also, probably need to add init.py in all conf modules.
update docs and examples with it.