Fix let function arguments not registering with the correct module name

[jfmengels]

@r-k-b noticed a bug in the lookup table implementation:

module A exposing (..)
import B exposing (someValue)

a =
    let
        b someValue =
            someValue <-- This one
    in
    someValue

The module name for the someValue tagged above (inside the body of b) was resolved a [ "B" ], which is incorrect and should be [], as the parameter name shadows the import.

This PR fixes the issue by correctly considering let functions to have a scope with their own variable bindings.