io/fs: move FileInfo, FileMode, PathError, ErrInvalid, ... from os to io/fs

io/fs: move FileInfo, FileMode, PathError, ErrInvalid, ... from os to io/fs

First step of creating the new io/fs package.

For #41190.

Change-Id: I1339b1abdd533b0f1deab283628088b2f706fb5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/243906
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>