Fix handling of AppliedType aliases in outerPrefix

[odersky]

Fixes #20184

The overlooked case was an AppliedType where we don't dealias correctly (i.e. at phase at most erasure).

[odersky]

This also seems to fix the underlying issue with parboiled2, which required a workaround previously.

[EugeneFlesselle]

Edited: It's is hard to come up with a simple test case causing the issue with match types, since they get simplified in Typer for now.

trait Color

trait A:
  type C
  type R = C match
    case Color => C

def outer3Local =
  object Inner:
    case class Rgb() extends Color

  val b = new A:
    type C = Inner.Rgb
    // compiles if definition of R is moved here

  new b.R() // Error: b.R is not a class type

This is not the same problem, but I think once it is fixed we would get the current issue in outerPrefix again.

[EugeneFlesselle]

superType will reduce AppliedTypes but not match types, so I think we could still trigger the issue.

[odersky]

@EugeneFlesselle ready to merge?

[EugeneFlesselle]

The previous example still fails, having reviewed the spec with sjrd, it is clear that it should indeed fail, for the same reasons as in #20259, even though similar cases with match types pass since they go though simplified.

It actually seems that the spec isn't clear about if type Id[X] = X should be accepted either, it addresses more specifically cases such as type A[X] = Array[X]. Either way, there is no simple was of differentiating them, so I think we can merge this.