benchmarks: move internal/stats to stats

the better to get at this useful code.

Change-Id: I844df856c016ccc25771bb4444b0ff06f5674bc8
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/321961
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
5 files changed