fix: ensure hatchling build-system for subfolder builds and fix source detection

• Fix source directory detection to correctly identify subfolders instead of project root
• Exclude virtual environments (.venv, venv) and build artifacts from file scanning
• Always use hatchling build-system for subfolder builds, replacing any existing setuptools configuration
• Add build-system section when using subfolder's pyproject.toml
• Add comprehensive tests for build-system section handling
• Update documentation to explain build-system section behavior

This fixes the issue where subfolder builds were failing due to:

• Incorrect source directory detection
• Virtual environment files being included in dependency analysis
• Missing or incorrect build-system configuration causing setuptools to be used instead of hatchling

Fixes build failures when running:
python-package-folder --publish azure --repository-url --version 1.0.0
from subfolder directories