gocore,core: move to internal package

The stability requirements for x/debug packages are unclear. Move
core and gocore to an internal package for now so we don't end up having
to maintain their APIs.

Change-Id: Idc8fba98409af12ccd801dad818b10917bd0a9bb
Reviewed-on: https://go-review.googlesource.com/116360
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
28 files changed