Extract read datasets from db to make it re-usable

[seitenbau-govdata]

Moves the code for reading datasets by the guid into a separate method to make it possible to re-use the method instead of using the method _get_existing_datasets (calls package_show additionally which is unnecessary in our case) or copy paste the code in our harvester implementation.

[metaodi]

LGTM

[seitenbau-govdata]

Thanks @metaodi !