Move scan plugins and data api definitions in their respective modules

We have been accumulating plugins in the scancode module as well as the declaration of the data structures returned by plugins in scancode/api.py.
This code should be moved to its respective modules for clarity.
This means that api.py should disappear... which would be a breaking change of the API: to handle this in an orderly fashion, the api.py will be marked as deprecated and will import from the code that has been moved in the short term to keep some backward compatibility.