Also, in 'go get' in GOPATH mode, report an error for package paths
that start with '-'.

Change-Id: Ic2575381aa2d093ba15c53b893bf2eaded8b6066
Reviewed-on: https://go-review.googlesource.com/c/go/+/181237
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>