io/fs, os: unify PathError.Path for dirFS.{ReadFile, ReadDir}

Fixes #64366

Change-Id: Ie78ab2cb9e11c0766665cd37fd7a26d36a1c24fa
GitHub-Last-Rev: 3cb3bb84a0ccccc88ba30258f52f699ac1364a7b
GitHub-Pull-Request: golang/go#64372
Reviewed-on: https://go-review.googlesource.com/c/go/+/544835


Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Jes Cok <xigua67damn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>