I have read through this PR a couple of times, and nothing stands out to me to me as a problem.

Apologies for the delay. Something was bothering me about thing change, but I couldn't quite figure out what exactly. However, I think I have :)

That thing is that build.Session API is still mostly trying to pretend it works on single packages, despite the fact that it loads the full dependency tree and stores it inside. However, in reality we are working on a full program, not a single package.

The dissonance is made worse by the fact that we sometimes synthesize parts of the program (like test packages), but that fact is almost unobservable.

A better API (for testability and understanding both) would be where each processing stage is (at least conceptually) a pure function with its input a full-program representation of the previous stage, and the return value, again, a full program of the next stage. Compare this with today, where are a just passing the reference to an entry package, and keep the majority of the information internally.

So, at some point, we probably should build on the idea you've started with Sources and implement types like LoadedProgram, AnalyzedProgram, CompiledProgram and GeneratedSource.

All that said... From practical perspective, you PR is a fine incremental step forward. Now that I understand what was bothering me, I also know that it's not an immediate concern, just a future direction.