[Bugfix] Migration workbench autoload generation.

I was noticing while making migrations in the workbench that the autoload dumping wasn't being applied to packages in the workbench.

This fixes that and also made it possible to override the working directory of the composer instance by passing the path to the `dumpAutoloads()` and `getProcess()` methods of `Illuminate\Foundation\Composer`.

The `findComposer()` method was only modified to include the entire path to composer.phar if it exists in the working path. (Can't be overridden)