Force-loading vendor autoload causes dependency conflicts

[ryanshoover]

We require our autoload to load, even if all the dependencies we need are already loaded. This can cause fatal errors if another WordPress plugin has already loaded a different version of our dependencies.
https://github.com/awslabs/aws-for-wordpress/blob/master/includes/class-amazonpolly.php#L162

For example, guzzle released a breaking change in 6.5.0 that changed the VERSION constant to MAJOR_VERSION. If both our guzzle version (6.4.1) and a later version of guzzle are loaded, then fatal errors can get triggered because of mixed dependencies.

To recreate

1. Set up a WordPress site that has a composer.json similar to below:

   {
   	"name": "wordpress/test",
   	"type": "project",
   	"description": "WordPress test instance",
   	"repositories": {
   		"wpackagist": {
   			"type": "composer",
   			"url": "https://wpackagist.org"
   		},
   		"s3uploads": {
   			"type": "vcs",
   			"url": "git@github.com:humanmade/S3-Uploads.git"
   		}
   	},
   	"require": {
   		"composer/installers": "^1.9",
   		"humanmade/s3-uploads": "2.2.2",
   		"wpackagist-plugin/amazon-polly": "4.2.1"
   
   	}
   }

2. In wp-config.php, add require vendor/autoload.php
3. Run wp plugin list
4. See a fatal error like

   PHP Fatal error:  Uncaught Error: Undefined class constant 'MAJOR_VERSION' in     /var/www/html/vendor/guzzlehttp/guzzle/src/Utils.php:121
   Stack trace:
   #0 /var/www/html/vendor/guzzlehttp/guzzle/src/functions.php(61): GuzzleHttp\Utils::defaultUserAgent()
   #1 /var/www/html/wp-content/plugins/amazon-polly/vendor/guzzlehttp/guzzle/src/Client.php(194): GuzzleHttp\default_user_agent()
   #2 /var/www/html/wp-content/plugins/amazon-polly/vendor/guzzlehttp/guzzle/src/Client.php(75): GuzzleHttp\Client->configureDefaults()
   #3 /var/www/html/wp-content/plugins/amazon-polly/vendor/aws/aws-sdk-php/src/Handler/GuzzleV6/GuzzleHandler.php(26): GuzzleHttp\Client->__construct()
   #4 /var/www/html/vendor/aws/aws-sdk-php/src/functions.php(279): Aws\Handler\GuzzleV6\GuzzleHandler->__construct()
   #5 /var/www/html/wp-content/plugins/amazon-polly/vendor/aws/aws-sdk-php/src/Sdk.php(417): Aws\default_http_handler()
   #6 /var/www/html/wp-content/plugins/amazon-polly/admin/AmazonAI-Common.php(284): Aws\Sdk->__construct()
   #7 /var/www/html/wp-content/plugins/amazon-polly in /var/www/html/vendor/guzzlehttp/guzzle/src/Utils.php on line 121