Restricting the Workbench->starter from looking beyond the packages vendor/autoload.php file

I was having a problem working on a package that shares a dependency on the predis/predis package with laravel/framework.  The problem is predis has a file within it called "autoload.php" that has `require` (not `require_once`) on a file that defines `Predis/Autoloader`.  So I was getting this error:

> Fatal error: Cannot redeclare class Predis\Autoloader in .../workbench/bkwld/library/vendor/predis/predis/lib/Predis/Autoloader.php on line 21

I think the intent of Starter::start() is to JUST load the composer generated autoload.php, so this commit enforces this.  It also has the benefit of REALLY speeding up page loading times (as reported in #553) when you have large (or multiple) packages in the workbench.