all: replace package ioutil with os and io in src

For #45557

Change-Id: I56824135d86452603dd4ed4bab0e24c201bb0683
Reviewed-on: https://go-review.googlesource.com/c/go/+/426257


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Auto-Submit: Ian Lance Taylor <iant@google.com>
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Reviewed-by: Ian Lance Taylor <iant@google.com>