[turbopack] Fix a small issue in the analyzer where we wouldn't skip assignments to free vars that were just identifiers (#82392)
Correctly skip assignments when replacing free variables
Drop an `Option<..>` for a parameter that is always supplied.
These are fixes i tripped over when attempting to compile time replace `module` and `exports`. That approach failed (see #82285 for what happened instead), but we can keep these cleanups.