Move source files into nested secpaste/ directory to fix module imports.
This allows the package to be properly installed with pip and imported as
'import secpaste' or 'python -m secpaste.cli'.
Changes:
- Move all Python files into secpaste/secpaste/ structure
- Add .gitignore for Python artifacts
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
