Bug: unused variables pass not taking into account `E_mod_in`

let x =
    let a = 1 in
    module B = struct 
        let y = a
    end in
    B.y

Here we get a warning that a is unused, because in unused.ml we don't rhs declrarions of E_mod_in

File "x.mligo", line 2, characters 8-9:
  1 | let x =
  2 |     let a = 1 in
  3 |     module B = struct 
:
Warning: unused variable "a".
Hint: replace it by "_a" to prevent this warning.