This commit fixes the missing `config/defaults.yaml` issue that caused
`pkgmgr --help` to fail with exit code 5 inside Nix builds and during
integration tests.
Changes:
- Added `config/__init__.py` to make the directory a proper Python package.
- Updated `pyproject.toml` to rely on setuptools package discovery.
- Extended `tool.setuptools.packages.find.include` to include both `pkgmgr*` and `config*`.
- Added `tool.setuptools.package-data` so `defaults.yaml` is included
in wheels, Nix builds, and PKGBUILD-derived installations.
- Removed the conflicting `tool.setuptools.packages` declaration.
This ensures that Nix's pypaBuildPhase can build the wheel successfully
and that `pkgmgr --help` no longer crashes due to missing default config.
Reference:
https://chatgpt.com/share/6935f05f-03d8-800f-a126-b3114cc116ec
