Turbopack: move assignment_scopes computation into ImportMap (#92835)
I'm moving all of the export-related logic into `EvalContext` (and thus `ImportMap`) so that `VarGraph` is only needed for computing the references, and not the exports.
The change in the snapshots is
```diff
-*anonymous function 170* (const after eval) = (...) => [file]
+*anonymous function 170* = (...) => [file]
```
which is fine, since computing the "is reassigned" data for something that isn't actually a variable was non-sensical anyway.