Data model subtask: client schema must be flattened

[rbalicki2]

Current behavior:

• When creating the nested client schema, we do not have server entities (e.g. Query)
• We look up server entities, in order to add a selectable to their selectables (e.g. Query.HomePage)
• This is not found; we emit a non-fatal diagnostic (which is ignored... oops) and continue.

Instead:

• We should directly create a flattened client schema, thus allowing us to create selectables with non-existent parent entities.
• That's fine, that should be validated at some point.