avoid weird autoloading side effects for psr 18 clients too (#202)

e.g. symfony httplug adapter triggers a warning if the php-http/httplug
is not available 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

@@ -17,6 +17,7 @@
 use Http\Client\HttpAsyncClient;
 use Http\Client\HttpClient;
 use Http\Client\Socket\Client as Socket;
+use Http\Discovery\ClassDiscovery;
 use Http\Discovery\Exception\NotFoundException;
 use Http\Discovery\MessageFactoryDiscovery;
 use Http\Discovery\Psr17FactoryDiscovery;
@@ -136,8 +137,11 @@ private static function getPsr18Candidates()
 
         // HTTPlug 2.0 clients implements PSR18Client too.
         foreach (self::$classes[HttpClient::class] as $c) {
+            if (!is_string($c['class'])) {
+                continue;
+            }
             try {
-                if (is_subclass_of($c['class'], Psr18Client::class)) {
+                if (ClassDiscovery::safeClassExists($c['class']) && is_subclass_of($c['class'], Psr18Client::class)) {
                     $candidates[] = $c;
                 }
             } catch (\Throwable $e) {