Remove minor usages of "stage" from Package Manager's runtime code

Problem/Motivation

📌 Clarify/replace 'stage' terminology in Package Manager Active renamed a lot of runtime classes and API surface in Package Manager, but there are still some places were "stage" is used in the code -- namely some local variables, and in tests. Let's fix all that here.

This will not touch any documentation; that's happening in 📌 Update all comments and documentation in Package Manager to refer to "sandboxes" and related terminology Postponed .