repo_management/convert.py:
Add private method `_transform_package_desc_to_output_package()` which
can be used to construct a `models.OutputPackage` from a
`models.PackageDesc` (and optionally a `models.PackageFiles`).
The method drops attributes from a dict representation of the
`models.PackageDesc` instance, that are not relevant for the
`models.OutputPackage`.

tests/test_convert.py:
Add parametrized test for `_transform_package_desc_to_output_package()`.