Make the project a proper Python package

[JohnGiorgi]

Installing this project from git using

pip install git+https://github.com/PathwayCommons/pathway-abstract-classifier.git

was failing because it wasn't structured as a python package. In general, you need:

• A top level-directory. Should be named after the "name" field in the pyproject.toml
• Each directory that contains python code that you expect to import from needs a file, __init__.py. This file can be blank.

I have made these changes and confirm that the package is now installable with:

pip install git+https://github.com/PathwayCommons/pathway-abstract-classifier.git@create-top-level-directory

@create-top-level-directory can be dropped once this is merged into main.