This is addressing feedback given on golang.org/cl/23052;
we do it in a separate CL to separate the functional from
the rename change.

ONAME was not used in the export data, but it's the natural node op
where we used OPACK instead. Renamed.

Furthermore, OPACK and ONONAME nodes are replaced by the type checker
with ONAME nodes, so OPACK nodes cannot occur when exporting type-checked
code. Removed a special-case for OPACK nodes since they don't appear.

Change-Id: I78b01a1badbf60e9283eaadeca2578a65d28cbd2
Reviewed-on: https://go-review.googlesource.com/23053
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>