function mapLaunchStoryboardResources (splashScreens, launchStoryboardImagesDir) {
    const platformLaunchStoryboardImages = mapLaunchStoryboardContents(splashScreens, launchStoryboardImagesDir);
    const pathMap = {};
    platformLaunchStoryboardImages.forEach(item => {
        if (item.target) {
            pathMap[item.target] = item.src;
        }
    });
    return pathMap;
}
Returns a dictionary representing the source and destination paths for the launch storyboard images
that need to be copied.

The resulting return looks like this:

    {
        'target-path': 'source-path',
        ...
    }

@param  {Array<Object>} splashScreens         splash screens as defined in config.xml for this platform
@param  {string} launchStoryboardImagesDir    project-root/Assets.xcassets/LaunchStoryboard.imageset/
@return {Object}