Use of private properties and methods for Dataset information retrieval

[jotelha]

When registering datasets via the Flask CLI, we make use of private properties and methods of dtoolcore's DataSet in two places, https://github.com/jic-dtool/dtool-lookup-server/blob/a92c1776f3204b77b226e29a1bc5505b756ac680/dtool_lookup_server/utils.py#L161 and https://github.com/jic-dtool/dtool-lookup-server/blob/a92c1776f3204b77b226e29a1bc5505b756ac680/dtool_lookup_server/utils.py#L169

We should avoid this outside of dtoolcore. See as well jic-dtool/dtoolcore#28