refactor the script into modules (#41)

* move the policy reading and validation to a separate module

* refactor the environment verification into a separate module

* move the `Release` class into a separate module

* add a function to fetch releases

* remove the unused `is_preview` function

* implement a draft of the cli

* define `__main__` and `__init__`

* typo

* implement the forgotten `find_policy_versions`

* remove the old script

* try using `PYTHONPATH` to run the action

* select validation

* change the unit tests

* replace the script test with multiple test modules

Author: keewis

action.yaml

@@ -38,4 +38,4 @@ runs:
         ENVIRONMENT_PATHS: ${{ inputs.environment-paths }}
         TODAY: ${{ inputs.today }}
       run: |
-        python ${{ github.action_path }}/minimum_versions.py --today="$TODAY" --policy="$POLICY_PATH" $ENVIRONMENT_PATHS
+        PYTHONPATH=${{github.action_path}} python -m minimum_versions validate --today="$TODAY" --policy="$POLICY_PATH" $ENVIRONMENT_PATHS