pbxProject hasFile returns false for existing file on Windows

We are running into an issue with this in our Expo CLI on Windows. After some investigation, it looks like there are some issues with Posix and Windows paths when detecting if files exists or not.

I [created a repository](https://github.com/bycedric/xcode-windows-hasfile-issue) with an example running on Ubuntu, MacOS and Windows that demonstrates this issue. The scenario it's testing is fairly simple:

```js
const PBX_PROJECT_PATH = path.resolve('ios', 'testios2.xcodeproj', 'project.pbxproj');
const SPLASH_SCREEN_PATH = path.join('testios2', 'SplashScreen.storyboard');

// Create a new pbx project instance
const project = xcode.project(PBX_PROJECT_PATH);
// Now parse the project
project.parseSync();

// Determine if "SplashScreen.storyboard" exists
const file = project.hasFile(SPLASH_SCREEN_PATH);
```

You see that this is done successfully on both [Linux](https://github.com/byCedric/xcode-windows-hasfile-issue/runs/1231850644?check_suite_focus=true#step:6:8) and [MacOS](https://github.com/byCedric/xcode-windows-hasfile-issue/runs/1231850677?check_suite_focus=true#step:6:8), but [fails on Windows](https://github.com/byCedric/xcode-windows-hasfile-issue/runs/1231850707?check_suite_focus=true#step:6:8).

There is a workaround, you have to force the `SPLASH_SCREEN_PATH` to always be a posix formatted path. That works on [Linux](https://github.com/byCedric/xcode-windows-hasfile-issue/runs/1231850644?check_suite_focus=true#step:5:8), [MacOS](https://github.com/byCedric/xcode-windows-hasfile-issue/runs/1231850677?check_suite_focus=true#step:5:8), _and_ [Windows](https://github.com/byCedric/xcode-windows-hasfile-issue/runs/1231850707?check_suite_focus=true#step:5:8).

```js
const SPLASH_SCREEN_PATH = path.join('testios2', 'SplashScreen.storyboard')
// Force the path to always be a posix formatted path
.replace(path.sep, path.posix.sep);
```