Add os.Root, a type which represents a directory and permits performing
file operations within that directory.
For #67002
Change-Id: I863f4f1bc320a89b1125ae4237761f3e9320a901
Reviewed-on: https://go-review.googlesource.com/c/go/+/612136
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
