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

Motivation and Context

The passes unused (for finding unused bindings) & muchused (for checking linearity for tickets) did not decend into the declarations of E_mod_in (location module in an expression).

Which caused unnecessary unused warnings for binding & no warnings in duplicate tickets.

Fixes #1672 (closed)

Description

This MR fixes these issues.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

For a contract like

let fst (x, _) = x

let snd (_, x) = x

let main (_ : unit * nat ticket) : operation list * nat ticket =
  let n = 10n in
  module B = struct 
    let ticket = Option.unopt (Tezos.create_ticket n n)
    let y = ticket, ticket
  end in
  [], Option.unopt (Tezos.join_tickets (fst B.y, snd B.y))

Before

 ligo.60 compile contract y.mligo 
File "y.mligo", line 6, characters 6-7:
  5 | let main (_ : unit * nat ticket) : operation list * nat ticket =
  6 |   let n = 10n in
  7 |   module B = struct 
:
Warning: unused variable "n".
Hint: replace it by "_n" to prevent this warning.

Error(s) occurred while type checking the contract:
Ill typed contract:
  ...

After

$ _build/install/default/bin/ligo compile contract y.mligo
File "y.mligo", line 8, characters 8-14:
  7 |   module B = struct 
  8 |     let ticket = Option.unopt (Tezos.create_ticket n n)
  9 |     let y = ticket, ticket
:
Warning: variable "ticket" cannot be used more than once.

File "y.mligo", line 9, characters 4-26:
  8 |     let ticket = Option.unopt (Tezos.create_ticket n n)
  9 |     let y = ticket, ticket
 10 |   end in
:
Warning: variable "B.y" cannot be used more than once.

Error(s) occurred while type checking the contract:
Ill typed contract:
  ...

Checklist:

• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).