Rename Dataset.source to avoid conflation with where an entity was first defined

[cpimhoff]

The dataset source: key is easily conflated with “where did the dataset come from”, as in the system where the resource was originally defined, such as from dbt or an app. This could be useful to explicitly encode in future for tools visualizing the flow of definitions through a complex data ecosystem.

This key defines the expression which constructs the table which serves as the backing store of the dataset. I’d suggest table: or table_expression: to clarify.