SI-7596 Curtail overloaded symbols during unpickling

In code like:

    object O { val x = A; def x(a: Any) = ... }
    object P extends O.x.A

The unpickler was using an overloaded symbol for `x` in the
parent type of `P`. This led to compilation failures under
separate compilation.

The code that leads to this is in `Unpicklers`:

    def fromName(name: Name) = name.toTermName match {
      case nme.ROOT     => loadingMirror.RootClass
      case nme.ROOTPKG  => loadingMirror.RootPackage
      case _            => adjust(owner.info.decl(name))
    }

This commit filters the overloaded symbol based on the
parameter list size when unpickling a singleton type.
That seemed a slightly safer place than in `fromName`.

Review by @gkossakowski

Points for review:
 - where do you think the right place is to filter the symbol? 
 - I chose `filter` over `suchThat` to defer errors in the case that the val no longer exists. But I didn't test that case, and don't really think the precise error mode is too important; things will blow up quickly either way.
 - is there a cleaner, flag-driven alternative to `paramss.isEmpty`?