Add a new custom attribute to compile units containing the package name
of the package (i.e. the name after the 'package' keyword), so that
debuggers can know it when it's different from the last segment
of the package path.

Change-Id: Ieadaab6f47091aabf2f4dc42c8524452eaa6715b
Reviewed-on: https://go-review.googlesource.com/c/go/+/163677
Run-TryBot: Alessandro Arzilli <alessandro.arzilli@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>