Add __init__ modules to enable subpackages

[jruizaranguren]

I have installed did-webs-resolver as a git dependency. The dkr entry point is found, but when trying to run the console tool it raises:

from dkr.core import didding
ModuleNotFoundError: No module named 'dkr.core'

This is probably due to dkr.core not including a __init__.py. I have added those to both core anddidcomm` subpackages.

[2byrds]

I haven't seen this issue but the change is welcome and hopefully only helpful. Thank you!