Right now they are in a deterministic order
but one that depends on the shape of the import graph.
Sort them instead.

Change-Id: Ia0c076a0d6677a511e52acf01f38353e9895dec2
Reviewed-on: https://go-review.googlesource.com/c/go/+/174527
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>