This allows both the runtime and the cmd/pprof code to use the package,
just like we do for internal/trace.

Change-Id: I7606977284e1def36c9647354c58e7c1e93dba6b
Reviewed-on: https://go-review.googlesource.com/32452
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>