Plugin should not try to prefetch repositories with unsupported protocols

[sergeyfedotov]

I have a problem with this plugin when https://github.com/naderman/composer-aws is also used. The problem is that s3:// protocol can't be correctly handled by the plugin.

[Composer\Downloader\TransportException]                                                                                                                                       
The "s3://***/packages.json" file could not be downloaded: Unable to find the wrapper "s3" - did you forget to enable it when you configured PHP?  
failed to open stream: No such file or directory                                                                                                                               
                                                                                                                                                                               

Exception trace:
() at phar:///usr/local/bin/composer/src/Composer/Util/RemoteFilesystem.php:482
Composer\Util\RemoteFilesystem->get() at phar:///usr/local/bin/composer/src/Composer/Util/RemoteFilesystem.php:101
Composer\Util\RemoteFilesystem->getContents() at phar:///usr/local/bin/composer/src/Composer/Repository/ComposerRepository.php:661
Composer\Repository\ComposerRepository->fetchFile() at phar:///usr/local/bin/composer/src/Composer/Repository/ComposerRepository.php:479
Composer\Repository\ComposerRepository->loadRootServerFile() at /root/.composer/vendor/hirak/prestissimo/src/ParallelizedComposerRepository.php:39
Hirak\Prestissimo\ParallelizedComposerRepository->prefetch() at /root/.composer/vendor/hirak/prestissimo/src/Plugin.php:132
Hirak\Prestissimo\Plugin->prefetchComposerRepositories() at /root/.composer/vendor/hirak/prestissimo/src/Plugin.php:76
Hirak\Prestissimo\Plugin->activate() at phar:///usr/local/bin/composer/src/Composer/Plugin/PluginManager.php:236
Composer\Plugin\PluginManager->addPlugin() at phar:///usr/local/bin/composer/src/Composer/Plugin/PluginManager.php:205
Composer\Plugin\PluginManager->registerPackage() at phar:///usr/local/bin/composer/src/Composer/Plugin/PluginManager.php:261
Composer\Plugin\PluginManager->loadRepository() at phar:///usr/local/bin/composer/src/Composer/Plugin/PluginManager.php:79
Composer\Plugin\PluginManager->loadInstalledPlugins() at phar:///usr/local/bin/composer/src/Composer/Factory.php:377
Composer\Factory->createComposer() at phar:///usr/local/bin/composer/src/Composer/Factory.php:569
Composer\Factory::create() at phar:///usr/local/bin/composer/src/Composer/Console/Application.php:314
Composer\Console\Application->getComposer() at phar:///usr/local/bin/composer/src/Composer/Console/Application.php:426
Composer\Console\Application->getPluginCommands() at phar:///usr/local/bin/composer/src/Composer/Console/Application.php:131
Composer\Console\Application->doRun() at phar:///usr/local/bin/composer/vendor/symfony/console/Application.php:122
Symfony\Component\Console\Application->run() at phar:///usr/local/bin/composer/src/Composer/Console/Application.php:100
Composer\Console\Application->run() at phar:///usr/local/bin/composer/bin/composer:54
require() at /usr/local/bin/composer:24