Added feature to not overwrite existing handlers if found in core application

[jaxwilko]

This PR introduces a simple change that allows for handlers to be embedded within the application and prevents vapor-cli from replacing them upon deployment.

Usage

In WinterCMS (a Laravel powered CMS) we utilise our own autoload.php which allows us to load our custom helpers before composer loads those provided by Laravel.

The core issue (being able to autoload files in a specific order) was recently closed by composer:
composer/composer#6768

Unfortunately when deploying with Vapor, the httpHandler.php is injected which calls:

require __DIR__.'/vendor/autoload.php';

This means that our application helpers are replaced by the core Laravel ones.