Add missing autoload calls

Until either php 7.4 or doctrine/persistence 2 is required, these will
be needed to make sure php recognises signatures using the old names as
compatible with signatures using the new names.
This is necessary for types defined outside Symfony that extend types
from Symfony and still use the old names in signatures of methods they
override.

More details at https://dev.to/greg0ire/how-to-deprecate-a-type-in-php-48cf

Fixes doctrine/DoctrineMongoDBBundle#616

Author: greg0ire

Form/Type/DoctrineType.php

@@ -282,3 +282,5 @@ public function reset()
         $this->choiceLoaders = [];
     }
 }
+
+interface_exists(ObjectManager::class);

Form/Type/EntityType.php

@@ -97,3 +97,5 @@ private function parameterToArray(Parameter $parameter): array
         return [$parameter->getName(), $parameter->getType(), $parameter->getValue()];
     }
 }
+
+interface_exists(ObjectManager::class);

Security/User/EntityUserProvider.php

@@ -153,3 +153,6 @@ private function getClassMetadata(): ClassMetadata
         return $this->getObjectManager()->getClassMetadata($this->classOrAlias);
     }
 }
+
+interface_exists(ObjectManager::class);
+interface_exists(ObjectRepository::class);