all: replace io/ioutil with io and os package

For golang/go#45557

Change-Id: Ib1e5d3116f04adaacb4d6b81898db255b3d5bd04
GitHub-Last-Rev: 6a580196cbdc8703799604a59e474f357e2a98e6
GitHub-Pull-Request: golang/benchmarks#3
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/430856
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
13 files changed