Necessary to move PathError to io/fs.

For #41190.

Change-Id: I05e87675f38a22f0570d4366b751b6169f7a1b13
Reviewed-on: https://go-review.googlesource.com/c/go/+/243900


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>