Better to have a separate file for each class clearly addressing
it's purpose with it's filename.

  Source base data model -> source.py
  SourceFactory -> _sourcefactory.py
  Element base data model -> element.py
  ElementFactory -> _elementfactory.py

Also moved the _plugin.py to _plugincontext.py, since the object
is infact _PluginContext, lets follow this rule all around for
code clarity.