With Go 1.16, the ioutil package was deprecated. In addition to being
moved into the os package, `ioutil.ReadDir()` was also changed to not
stat(3P) all dir entries anymore. As a result, the caller now has to
do so manually. This is a performance improvement in some cases where
the caller didn't require any of the file information, but really only
wanted to read the directory's entries.

Adapt trivial usecases of `ioutil.ReadDir()` which do not require this
information with usage of `os.ReadDir()`. This leaves a few callsites of
the old `ioutil.ReadDir()` function for future conversion.