SI-7233 Account for aliased imports in eta expansion.

Buggy:

    treeCopy.Select(sel, sel.qual, sel.name) setSymbol null
    Select(sel, sel.qual, sel.name)

Okay:

    treeCopy.Select(sel, sel.qual, sel.name)
    Select(sel, sel.qual, sel.symbol.name) // but doesn't copyAttrs!

It is an easy mistake to make, I've found one more occurance:

  def foo(a: Any) = { import a.{toString => toS}; toS }

    error: uncaught exception during compilation: scala.reflect.internal.FatalError
    scala.reflect.internal.FatalError: class Object does not have a member toS
        at scala.reflect.internal.Definitions$DefinitionsClass.scala$reflect$internal$Definitions$DefinitionsClass$$fatalMissingSymbol(Definitions.scala:1028)

A followup commit will address that problem.
SI-7233 Account for aliased imports in Erasure

When we discard the fiction of `scala.Any`.