[PROOF-OF-CONCEPT/DO NOT MERGE] Avoid forcing type aliases to get their typeParams

[smarter]

This is a strawman proposal for fixing #970, before this commit, the
following failed with a cyclic reference error:

dotc B.scala\
./scala-scala/src/library/scala/collection/immutable/Seq.scala\
./scala-scala/src/library/scala/package.scala\
./scala-scala/src/library/scala/collection/GenSeqLike.scala\
./scala-scala/src/library/scala/collection/SeqLike.scala\
./scala-scala/src/library/scala/collection/generic/GenSeqFactory.scala

where B.scala is defined as:

object B{
  def main(args: Array[String]): Unit = {
    val s = List(1,2,3)
    ()
  }
}

Here's what I think is happening:

1. Unpickling a Scala2 TypeRef requires us to perform type application.
   (Scala2Unpickler#readType)
2. To apply a TypeAlias to some arguments, we first need to determine its
   type parameters.
   (TypeApplications#appliedTo and TypeApplications#typeParams)
3. To do this, we look at the type parameters of the underlying type of
   the TypeAlias, this requires completing the TypeAlias.
4. If the TypeAlias is defined in a source tree and not unpickled, this
   forces us to typecheck its right-hand side.
   (Namer#Completer#typeSig)
5. In turns, this forces various classes to be completed, which might
   themselves refer indirectly to type aliases, forcing even more
   stuff.

This commit is a hacky way to avoid 3. by only completing the type parameters
of a type alias instead of completing the whole type alias when we don't
need it. Let me know if you think the basic idea is sound or not and if
you can think of a nicer way to implement it!

CC @odersky

[odersky]

Superseded by #1027