fix mutation to imported variable

[tanhauhau]

Fixes #4555

• Import statements were by default hoistable, making it unavailable in the context.
• Make import declared variable non-hoistable if it is mutated somewhere. (import declared vars are read-only, cant be reassigned)
  • or should i created another flag for the var instead of changing the behavior of hoistable ?
  • import statements are hoisted by default, so changing hoistable seemed safe.

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[Conduitry]

I think using hoistable for this makes sense - it's not one of the fields we expose in the vars array, and so we don't need to worry about changing a public API or anything.