[Bridge\ProxyManager] isProxyCandidate() does not take into account interfaces

When using factories it's common best practice to use interface as class name, especially in cases
where you know impl can differ. Before this fix ProxyManager did not allow these to be lazy.

In our case this has lead several to hard to debug issues on classes we need to mark as lazyi
and often a need to add lazy on decorators if there are any or other workarounds.
As we have had this issue, and still have on both 2.8 and 3.4 this is opened against 2.8.

src/Symfony/Bridge/ProxyManager/LazyProxy/PhpDumper/ProxyDumper.php

@@ -47,7 +47,7 @@ class ProxyDumper implements DumperInterface
      */
     public function isProxyCandidate(Definition $definition)
     {
-        return $definition->isLazy() && ($class = $definition->getClass()) && class_exists($class);
+        return $definition->isLazy() && ($class = $definition->getClass()) && (class_exists($class) || interface_exists($class));
     }
 
     /**

src/Symfony/Bridge/ProxyManager/Tests/LazyProxy/PhpDumper/ProxyDumperTest.php

@@ -15,6 +15,7 @@ use PHPUnit\Framework\TestCase;
 use Symfony\Bridge\ProxyManager\LazyProxy\PhpDumper\ProxyDumper;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\Definition;
+use Symfony\Component\DependencyInjection\LazyProxy\PhpDumper\DumperInterface;
 
 /**
  * Tests for {@see \Symfony\Bridge\ProxyManager\LazyProxy\PhpDumper\ProxyDumper}.
@@ -137,6 +138,7 @@ class ProxyDumperTest extends TestCase
         $definitions = [
             [new Definition(__CLASS__), true],
             [new Definition('stdClass'), true],
+            [new Definition(DumperInterface::class), true],
             [new Definition(uniqid('foo', true)), false],
             [new Definition(), false],
         ];