Pickler/Unpickler dealias too eagerly

[allanrenucci]

object Trees {
  class PackageDef
  class ValDef
}

class ReplCompiler {
  type PackageDef = Trees.PackageDef
  type ValDef = Trees.ValDef

  def errors[A]: A = ???
  def success: ValDef = ???

  def typeCheck(cond: Boolean): ValDef =
    if (cond) success else errors // failure goes away with `errors[ValDef]`
}

> dotc ReplCompiler.scala -Ytest-pickler -Xprint-types
Exception in thread "main" class dotty.tools.dotc.reporting.diagnostic.messages$Error at ?: pickling difference for class ReplCompiler in ReplCompiler.scala, for details:
                   |
                   |  diff before-pickling.txt after-pickling.txt
	at dotty.tools.dotc.reporting.Reporting.error(Reporter.scala:96)
...

> diff before-pickling.txt after-pickling.txt 
27c27
<       :ReplCompiler.this.ValDef>
---
>       :Trees.ValDef>

Minimised from #4915

[panacekcz]

Let me add another reproduction, loosely based on tests/pos/t8237.scala:

object TI {
  class LA
  type LB[T] = LA
  trait TC[T]
  def fTt[E[X] <: LB[X]](tt: TC[E[Int]]) = 1
  def tc[T]: TC[T] = ???
  fTt(tc)
}

dotc -Ytest-pickler TI.scala:

exception occurred while compiling TI.scala
class dotty.tools.dotc.reporting.diagnostic.messages$Error at ?: pickling difference for module class TI$ in TI.scala, for details:

diff before-pickling.txt after-pickling.txt

15c15
<     TI.fTt[TI.LB](TI.tc[TI.LB[scala.Int]])
---
>     TI.fTt[TI.LB](TI.tc[TI.LA])