Remove `TypeReference`s

[laerreal]

First patches do some refactoring for final code to be more pretty. "source: remove TypeReference" does the main things. Last patch does some cleanup.

v2

• remove AddTypeRefToDefinerException
• remove unused imports from "source/source_file.py"
• fixup TestReferencingToTypeInAnotherHeader by adding foreign types to to Source's types registry.
• TypeFixerVisitor breaks on first foreign type (previously it stopped on a TypeReference).
• some refactoring
• fixup typos