repo_management/operations.py:
Add `create_db_from_json_files()` to create a repository database from
JSON files in a directory.
Change `db_file_as_models()` to rely on
`models.PackageDesc.get_output_packages()` instead of
`convert._transform_package_desc_to_output_package()` and add a missing
parameter (base) to the instantiation of `models.OutputPackageBase()`.

tests/test_operations.py:
Add the two fixtures `dummy_json_files_in_dir()` and `empty_file()` to
create a few (valid) dummy input JSON files in a temporary directory and
an empty file (respectively).
Add a test for `create_db_from_json_files()`.