repo_managament/*:
Remove the obsolete
`convert._transform_package_desc_to_output_package()` (the functionality
is now covered by pydantic models directly).
Change all relevant methods to be async.
Change file open commands to make use of aiofiles.

tests/*:
Remove tests for the obsolete
`convert._transform_package_desc_to_output_package()`.
Change all tests for async methods to test using pytest-asyncio.

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()`.

repo_management/files.py:
Add the private method `_extract_db_member_package_name()` which can be
used to extract the package name from the name of a member of repository
database.
Rename `_read_db_file_member()` to `_db_file_member_as_model()` and
change the method to yield a `models.RepoDbMemberData` instead of a
`tarfile.TarInfo` to provide actual data (and required context) to
callers of the method.

tests/test_file.py:
Add parametrized test for `_extract_db_member_package_name()`.
Change test for `_db_file_member_as_model()` to assert the correct
return type.

repo_management/files.py:
Add iterator method `_read_db_file_member()` to yield the members of a
provided tarfile.TarFile.
The members are filtered by name using a regular expression, which can
be overridden.

tests/test_file.py:
Change `create_db_file()` to also create 'desc' and 'files' files below
the created temporary directories.
Assert, that all tested methods return a value.
Add test for `_read_db_file_member()`.

repo_management/files.py:
Add `_read_db_file()` to open a db file and return it as
tarfile.Tarfile. The method assumes gzip-compressed databases by
default, but can be used with other compression algorithms.

tests/test_file.py:
Add fixtures and tests for `_read_db_file()`.