Introduce a new noder type to transform package syntax's AST into gc's
Node tree. Hidden behind a new -newparser flag.

Change-Id: Id0e862ef6196c41533876afc4ec289e21d422d18
Reviewed-on: https://go-review.googlesource.com/27198
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>