refactor: Make the project importable as a module

[skoenig]

This PR introduces changes to make the project importable as a Python module and introduces more modern packaging standards.

• Moved all project files into the git_metrics subfolder to create a proper Python module structure.
• Updated the import statements in various modules to use explicit relative imports.
• Introduced pyproject.toml file for PEP 517/518 standards, and added setup.py to ensure compatibility with both traditional and modern packaging tools.
• Added entry points in setup.py for command-line scripts, making calculate_four_metrics.py and git_metrics.py accessible.

[skoenig]

Background is: with these changes git-metrics can be used to instrument CI/CD pipelines to fetch the four DevOps metrics for reporting, enhancing the project's versatility.