src/Composer/Plugin/PluginManager.php
public function loadInstalledPlugins()
    {
        if ($this->disablePlugins) {
            return;
        }

        $repo = $this->composer->getRepositoryManager()->getLocalRepository();
        $globalRepo = $this->globalComposer ? $this->globalComposer->getRepositoryManager()->getLocalRepository() : null;
        if ($repo) {
            $this->loadRepository($repo);
        }
        if ($globalRepo) {
            $this->loadRepository($globalRepo);
        }
    }
public function loadInstalledPlugins()
    {
        if ($this->disablePlugins) {
            return;
        }

        $repo = $this->composer->getRepositoryManager()->getLocalRepository();
        $globalRepo = $this->globalComposer ? $this->globalComposer->getRepositoryManager()->getLocalRepository() : null;
        if ($repo) {
            $this->loadRepository($repo);
        }
        if ($globalRepo) {
            $this->loadRepository($globalRepo);
        }
    }
Loads all plugins from currently installed plugin packages