go/*: use sort.Slice to simplify some code

go/*: use sort.Slice to simplify some code

Skip the ones that have multiple uses for now. Also had to rename the
importComment variable as it shadowed the top-level func by the same
name.

Change-Id: I796285aa7b4fdf2c39e652666390427d37b063ee
Reviewed-on: https://go-review.googlesource.com/63150
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>