CakePHP 3.5: Cannot access Plugin Classes after installing it via Composer - composer-php

Question

So I have a CakePHP 3 project and want to load FluentDOM, a PHP plugin not specifically written for CakePHP.
According to both software documentations, Composer is the way to go. In my understanding, all I would have to do is the following:
run composer require fluentdom/fluentdom in powershell
run composer require fluentdom/selectors-phpcss in powershell
OR
add the following to composer.json in the project's root directory:
"require": {
"fluentdom/fluentdom": "^7.0",
"fluentdom/selectors-phpcss": "^1.1"
}
run composer update in powershell
Both ways will install the desired plugins to vendor/fluentdom/{pluginname}/ as expected, but /vendor/cakephp-plugins.php won't include them, as implied by CakePHP's plugin installation manual.
The following attempt to load either plugin in a controller by writing
use Cake\Core\Plugin;
Plugin::load('fluentdom/fluentdom');
Plugin::load('fluentdom/selectors-phpcss');
would cause an exception that the desired plugins were not found in plugins/ :
Make sure your plugin fluentdom/fluentdom is in the {absolute project path}\plugins\ directory and was loaded
-- Which is already odd, because Composer wouldn't install anything there to begin with.
I found that I might get around this issue by manually extending vendor/cakephp-plugins.php to include the correct paths:
'fluentdom/fluentdom' => $baseDir . '/vendor/fluentdom/fluentdom/',
'fluentdom/selectors-phpcss' => $baseDir . '/vendor/fluentdom/selectors-phpcss/'
(However, that doesn't seem the way to go, because this file is auto-generated and overwritten by Composer after every update.)
And even then, the final issue still persists: although the plugins seem to be loaded successfully (confirmed by running Plugin::loaded()), I'd finally get the following exception when trying to access FluentDOM's classes as described in their wiki:
$document = new FluentDOM\DOM\Document();
Class 'App\Controller\FluentDOM\DOM\Document' not found
Does the plugin miss out on having its' autoload executed?
Even extending the line in my controller to Plugin::load('fluentdom/fluentdom', ['autoload' => true]);, but doesn't seem to help either; according to CakePHP's doc, that shouldn't be necessary anyway.
So what am I missing?

Answer 1

Found it! First of all, I had the false presumption that Plugins and Vendor Packages are more or less the same: they are not; thanks to Greg Schmidt for pointing this out in the question's comments.
The issue was in the line of how I tried to access FluentDOM's class. While
$document = new FluentDOM\DOM\Document();
worked in a standalone php file, it didn't within the Cake project; I was missing a backslash:
$document = new \FluentDOM\DOM\Document();
So, the entire path of actions to load a Vendor Package is merely:
run composer require fluentdom/fluentdom in powershell
run composer require fluentdom/selectors-phpcss in powershell
Use the new classes right away with $document = new \FluentDOM\DOM\Document();
No further steps required. Side note: Composer seems to refresh autoload config after installing a vendor file with composer require {vendor}/{package}, but in case it doesn't, or autoload config is messed up from earlier experiments, composer dumpautoload should fix it.