Fix 'Unbound module type' errors that occurred when trying to async import modules

[mediremi]

Closes #7173

This fixes two cases where async module import previously failed:

// Case 1: return type constraint
let f8 = async (): int => {
  module MQ = await Belt.MutableQueue
  1
}

// Case 2: import in switch case
let f9 = async value => {
  switch value {
  | Some(_) =>
    module HashMapInt = await Belt.HashMap.Int
    ()
  | None => ()
  }
}

The fix was to handle Pexp_constraint and Pexp_match in spelunk_vbs (spelunk Parsetree.value_bindings - finds module bindings).

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commit: d6a8395