Check Windows case insensitive app name

On Windows if we run `node-debug app` and real file name is App.js,
we have two identical files in resources tree (app.js and App.js),
but we can't set breakpoints in App.js.
On non-Windows platforms, `fs.readdir` is not necessary and may add extra time to startup. Can you please rework this patch so that `fs.readdir` is executed only on Windows?