winfsnotify: deprecate package and isolate it to its own module

For golang/go#51447.

Change-Id: I3266073d85e52aa48ba6c8578b2b9b9e26486060
Reviewed-on: https://go-review.googlesource.com/c/exp/+/399834
Reviewed-by: Russ Cox <[email protected]>

Author: Bryan C. Mills

winfsnotify/go.mod

@@ -0,0 +1,3 @@
+module golang.org/x/exp/winfsnotify
+
+go 1.11

winfsnotify/winfsnotify.go

@@ -7,6 +7,11 @@
 
 // Package winfsnotify allows the user to receive
 // file system event notifications on Windows.
+//
+// Deprecated: winfsnotify has been unmaintained since 2016 (see
+// https://go.dev/issue/51447). Forks of this package with more recent
+// maintenance and cross-platform support can be found at
+// https://pkg.go.dev/search?q=fsnotify.
 package winfsnotify // import "golang.org/x/exp/winfsnotify"
 
 import (