cmd/compile: minor cleanup to import loading

cmd/compile: minor cleanup to import loading

Briefly document what the importfoo functions do.

Get rid of importsym's unused result parameter.

Get rid of the redundant calls to importsym(s, OTYPE)
after we've already called pkgtype(s).

Passes toolstash -cmp.

Change-Id: I4c057358144044f5356e4dec68907ec85f1fe806
Reviewed-on: https://go-review.googlesource.com/21498
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>