src/Composer/Installer/LibraryInstaller.php
public function isInstalled(InstalledRepositoryInterface $repo, PackageInterface $package)
    {
        if (!$repo->hasPackage($package)) {
            return false;
        }

        $installPath = $this->getInstallPath($package);

        if (is_readable($installPath)) {
            return true;
        }

        return (Platform::isWindows() && $this->filesystem->isJunction($installPath)) || is_link($installPath);
    }
public function isInstalled(InstalledRepositoryInterface $repo, PackageInterface $package)
    {
        if (!$repo->hasPackage($package)) {
            return false;
        }

        $installPath = $this->getInstallPath($package);

        if (is_readable($installPath)) {
            return true;
        }

        return (Platform::isWindows() && $this->filesystem->isJunction($installPath)) || is_link($installPath);
    }
{@inheritDoc}