Refactored file organization

[cicirello]

Summary

Refactored file organization, including (among others) isolating the action's entrypoint code to its own file. The purpose is to ease maintainability, as well as in support of future implementation of #40 and also relates to #62.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Improvements to existing code, such as refactoring or optimizations (non-breaking)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.