Bindings in structural self types are not picked up correctly.

The following typechecks:
```scala
trait TS { type T = Int }
trait T { self: TS =>
  type T
  def lift(x: Int): T = x
}
```
But this variant doesn't:
```scala
trait T { self: { type T = Int } =>
  type T
  def lift(x: Int): T = x  // error: expected T, found: Int
}
```
The two should behave the same.
