src/Composer/Installer/InstallationManager.php
public function isPackageInstalled(InstalledRepositoryInterface $repo, PackageInterface $package)
    {
        if ($package instanceof AliasPackage) {
            return $repo->hasPackage($package) && $this->isPackageInstalled($repo, $package->getAliasOf());
        }

        return $this->getInstaller($package->getType())->isInstalled($repo, $package);
    }
public function isPackageInstalled(InstalledRepositoryInterface $repo, PackageInterface $package)
    {
        if ($package instanceof AliasPackage) {
            return $repo->hasPackage($package) && $this->isPackageInstalled($repo, $package->getAliasOf());
        }

        return $this->getInstaller($package->getType())->isInstalled($repo, $package);
    }
Checks whether provided package is installed in one of the registered installers.

@param InstalledRepositoryInterface $repo    repository in which to check
@param PackageInterface             $package package instance

@return bool