all: replace io/ioutil with io and os package

For golang/go#45557

Change-Id: I447530cc66896aef7a8d528ccb8d095b80e3cf47
GitHub-Last-Rev: 5f385ff46487ac318bd1147cdbbd26bb0ffd0426
GitHub-Pull-Request: golang/crypto#230
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/430797
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

openpgp/write.go

@@ -402,7 +402,7 @@ func (s signatureWriter) Close() error {
 	return s.encryptedData.Close()
 }
 
-// noOpCloser is like an ioutil.NopCloser, but for an io.Writer.
+// noOpCloser is like an io.NopCloser, but for an io.Writer.
 // TODO: we have two of these in OpenPGP packages alone. This probably needs
 // to be promoted somewhere more common.
 type noOpCloser struct {