Wunused: fix import false positive in constructors

[szymon-rd]

Fix #19252

In theory, it adds a new false negative instead, but the tree after typer contains the def of val from constructor, so the false pos would be present regardless:

package <empty> {
  package foo {
    trait D1() extends Object {}
  }
  final lazy module val Bug: Bug = new Bug()
  final module class Bug() extends Object() { this: Bug.type =>
    import foo.D1
    class Cl(d1: foo.D1) extends Object() {
      private[this] val d1: foo.D1
      import foo.*
    }
  }
}

[som-snytt]

Superseded, PR at ticket.