Split large source files into a proper package structure

Moodle Tools is currently implemented in two large python files, which makes the project hard to overlook and expand. We therefore want to split the code base into a proper module structure with dedicated files for classes, etc. It could look roughly like this:

moodle_tools
|-questions
  |-__init__.py
  |-question.py (abstract base class for all other questions)
  |-<question type 1>.py
  |-<question type 2>.py
  |-...
|-analysis
  |-__init__.py
  |-<TBD>
|-__init__.py
|-utils.py