Is the idea that python_sources contains the meta data for a group of python files that form a coherent "thing" (now known as a library)

That's the thing, there need not be any coherence at all. It could be a single source file, or all your source files, or 5 of them etc. All it is is metadata over n number of Python files, which all share the same explicit metadata like

interpreter_constraints

. Right now, we incorrectly suggest they must be coherent