os: don't fallback to the Stat slow path if file doesn't exist on Windows

os: don't fallback to the Stat slow path if file doesn't exist on Windows

os.Stat and os.Lstat first try stating the file without opening it. If
that fails, then they open the file and try again, operations that tends
to be slow. There is no point in trying the slow path if the file
doesn't exist, we should just return an error immediately.

This CL makes stating a non-existent file on Windows 50% faster:

goos: windows
goarch: amd64
pkg: os
cpu: Intel(R) Core(TM) i7-10850H CPU @ 2.70GHz
                │   old.txt    │                new.txt                 │
                │    sec/op    │    sec/op     vs base                  │
StatNotExist-12   43.65µ ± 15%   20.02µ ± 10%  -54.14% (p=0.000 n=10+7)

                │  old.txt   │             new.txt              │
                │    B/op    │    B/op     vs base              │
StatNotExist-12   224.0 ± 0%   224.0 ± 0%  ~ (p=1.000 n=10+7) ¹
¹ all samples are equal

                │  old.txt   │             new.txt              │
                │ allocs/op  │ allocs/op   vs base              │
StatNotExist-12   2.000 ± 0%   2.000 ± 0%  ~ (p=1.000 n=10+7) ¹

Updates #72992.

Change-Id: Iaeb9596d0d18e5a5a1bd1970e296a3480501af78
Reviewed-on: https://go-review.googlesource.com/c/go/+/671458
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jake Bailey <jacob.b.bailey@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>