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.