use different variable names for SDK and ORM objects

[redshiftzero]

Minor developer confusion issue for maintainability:

We have objects with the same name (e.g. source), that are in some cases SDK objects, and in others ORM objects (i.e. rows from the client database).

We should make these variables names distinct, i.e. instead of source, we use source_db_object and source_sdk_object to be crystal clear what they are in each case.

Thoughts?

[redshiftzero]

I'm closing this because type annotations make this clearer (we've been importing the SDK objects as e.g. SDKSource for the purpose of type annotations)