This is not necessary for reproduceability but it removes
differences due to imported package order between compiles
using textual vs binary export format. The packages list
tends to be very short, so it's ok doing it always for now.

Guarded with a documented (const) flag so it's trivial to
disable and remove eventually.

Also, use the same flag now to enforce parameter numbering.

Change-Id: Ie05d2490df770239696ecbecc07532ed62ccd5c0
Reviewed-on: https://go-review.googlesource.com/22445
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>