cmd/compile: cleanup import logic slightly

cmd/compile: cleanup import logic slightly

Use bio.Reader. Include newline character in the expected string value
instead of truncating it. Get rid of weird "empty archive" check.

Passes toolstash-check.

Change-Id: I16e42542db4827e6ee3644b9a5540a4a30b9bc41
Reviewed-on: https://go-review.googlesource.com/107620
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>