Mark tuples and lambda case in annotations for better error messages

Follow-up from #1225

The PR ^ adds detection of tuples and lambda case for better errors in a very ad-hoc way, we should probably replace that with a more principled approach.
One possibility is `findPrelude` for the definition of the tuple _type_, but it doesn't scale well for the other issues.

Another way of resolving it is to have some `Desugared` annotation which can tell us stuff like "oh, this `__arg` is just a desugared lambda-case!"

Start with follow-up issue #1225 and the existing ad-hoc detection for tuples and lambda-case described there. Compare the `findPrelude` approach with a principled `Desugared` annotation, and define completion as consistently identifying desugared constructs so error messages can distinguish tuple types and lambda-case code.

Written by the indexing model from the issue text.