ioutil package was deprecated in Go 1.16, replace all uses with their
respective replacements.
This patch was generated with a combination of `gofmt -r`, `eg`, and
manually (for `ioutil.ReadDir`).
We've accumulated a few linter issues around comments and a couple of
variable names.
While none of them is major, this patch cleans them up so it's easier to
go through the linter output, and we can start being more strict about
it.
This patch moves the test helper binaries to a "one directory per
helper" layout, and also makes them to be ignored in the coverage build
instead of all builds.
With this change, "go build ./..." will build all binaries including the
test helpers, which helps make sure that module manage automation also
considers them. In particular, this makes "go mod tidy" work fine.
