refactor: restructure project into a proper Python package layout (v2026.5.6)

[advaitpatel]

Summary

This PR overhauls the DockSec project structure, moving from a flat layout to a standard Python package structure. These changes make the codebase cleaner, more maintainable, and production-ready, aligning with industry standards for Python open-source projects.

Key Changes

• Package Restructuring: Created a core docksec/ package directory and moved all logic files (config.py, docker_scanner.py, utils.py, etc.) into it.
• CLI Entry Point: Renamed the main entry point to docksec/cli.py and updated setup.py to point to the new location.
• Code Cleanup: Removed redundant main.py and Windows metadata artifacts (*:Zone.Identifier) that were cluttering the repository.
• Imports & Paths: Updated all internal imports to absolute package imports and fixed file path logic for loading templates within the package.
• Packaging & Distribution: Updated setup.py, MANIFEST.in, and pyproject.toml to ensure proper distribution and inclusion of all package data.
• Documentation: Updated README.md and CONTRIBUTING.md to reflect the new structure and added a CHANGELOG.md entry for this overhaul.

Test Plan

• CLI Verification: Confirmed that python3 -m docksec.cli --help works correctly.
• Automated Tests: Ran the full test suite using pytest. All 17 unit and integration tests passed successfully after the restructuring.