symfony httplug adapter needs php-http httplug to be installed

if we don't check for the httplug client interface, we trigger a
warning:
https://github.com/symfony/http-client/blob/290eb481973b4984eb59377585f6afbc65a0f645/HttplugClient.php#L44

Author: dbu

src/Psr18ClientDiscovery.php

@@ -24,7 +24,7 @@ public static function find()
         try {
             $client = static::findOneByType(ClientInterface::class);
         } catch (DiscoveryFailedException $e) {
-            throw new \Http\Discovery\Exception\NotFoundException('No PSR-18 clients found. Make sure to install a package providing "psr/http-client-implementation". Example: "php-http/guzzle6-adapter".', 0, $e);
+            throw new \Http\Discovery\Exception\NotFoundException('No PSR-18 clients found. Make sure to install a package providing "psr/http-client-implementation". Example: "php-http/guzzle7-adapter".', 0, $e);
         }
 
         return static::instantiateClass($client);

src/Strategy/CommonClassesStrategy.php

@@ -75,14 +75,14 @@ final class CommonClassesStrategy implements DiscoveryStrategy
             ['class' => SlimUriFactory::class, 'condition' => [SlimRequest::class, SlimUriFactory::class]],
         ],
         HttpAsyncClient::class => [
-            ['class' => SymfonyHttplug::class, 'condition' => [SymfonyHttplug::class, Promise::class, RequestFactory::class, [self::class, 'isPsr17FactoryInstalled']]],
+            ['class' => SymfonyHttplug::class, 'condition' => [SymfonyHttplug::class, HttpClient::class, Promise::class, RequestFactory::class, [self::class, 'isPsr17FactoryInstalled']]],
             ['class' => Guzzle7::class, 'condition' => Guzzle7::class],
             ['class' => Guzzle6::class, 'condition' => Guzzle6::class],
             ['class' => Curl::class, 'condition' => Curl::class],
             ['class' => React::class, 'condition' => React::class],
         ],
         HttpClient::class => [
-            ['class' => SymfonyHttplug::class, 'condition' => [SymfonyHttplug::class, RequestFactory::class, [self::class, 'isPsr17FactoryInstalled']]],
+            ['class' => SymfonyHttplug::class, 'condition' => [SymfonyHttplug::class, HttpClient::class, RequestFactory::class, [self::class, 'isPsr17FactoryInstalled']]],
             ['class' => Guzzle7::class, 'condition' => Guzzle7::class],
             ['class' => Guzzle6::class, 'condition' => Guzzle6::class],
             ['class' => Guzzle5::class, 'condition' => Guzzle5::class],