undo CL 142770043 / b9df84379b1b

Benchmark builds prior to CL 134570043 will fail.

««« original CL description
go.benchmarks: fix garbage benchmark after src/pkg change

Possibly this code should use go/imports, but this should get the benchmark working again.

LGTM=bradfitz
R=dvyukov, adg, bradfitz
CC=golang-codereviews
https://golang.org/cl/142770043
»»»

TBR=dvyukov
R=golang-codereviews
CC=golang-codereviews
https://golang.org/cl/141290043