repo_management/files.py:
Add `_json_files_in_directory()` yield a list of files with a .json
suffix in a directory.
Add `_read_pkgbase_json_file()` to read a JSON file representing a
pkgbase and return an `OutputPackageBase` model.
Add `_write_db_file()` to open a tar file for writing.
Add `_stream_package_base_to_db()` to stream a `models.OutputPackage()`
to a tar file (which represents a .db or a .files repository database).

tests/test_files.py:
Add tests for `_json_files_in_directory()`, `_read_pkgbase_json_file()`,
`_write_db_file()` and `_stream_package_base_to_db()`.