Prefer globally installed packages in PluginManager autoload

When a package is installed globally and in the current project, PluginManager's autoload is built with the current project version which will break if you're going to update that package:

```
Loading composer repositories with package information
Updating dependencies (including require-dev)
  - Removing aws/aws-sdk-php (2.4.11)
  - Installing aws/aws-sdk-php (2.6.12)
    Downloading: connection...PHP Fatal error:  Class 'Aws\S3\S3Client' not found in /srv/home/a_samson/.composer/vendor/naderman/composer-aws/src/Naderman/Composer/AWS/AwsClient.php on line 161

Fatal error: Class 'Aws\S3\S3Client' not found in /srv/home/a_samson/.composer/vendor/naderman/composer-aws/src/Naderman/Composer/AWS/AwsClient.php on line 161
```

This PR makes the globally installed version preferred.