Add the Localize function, which takes an io/fs slash-separated path
and returns an operating system path.
Localize returns an error if the path cannot be represented on
the current platform.
Replace internal/safefile.FromFS with Localize,
which serves the same purpose as this function.
The internal/safefile package remains separate from path/filepath
to avoid a dependency cycle with the os package.
Fixes#57151
Change-Id: I75c88047ddea17808276761da07bf79172c4f6fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/531677
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
