Name _ was shadowed not raised for imported values

[safareli]

For example eq is imported from Prelude and you have:

case "foo" of
  "foo" -> "oof"
  eq -> eq

You will not get warning Name eq was shadowed but if you do:

let qq = "ff" in case "foo" of
  "foo" -> "oof"
  qq -> qq

You will get Name qq was shadowed.

[hdgarrood]

Note that we are considering removing the shadowed name warning entirely: #3375. It might be worth coming to a decision about that before implementing this.