Issue #3387656: Package name schema regex is not validated

package_manager/config/schema/package_manager.schema.yml

@@ -4,7 +4,7 @@ package_name:
   constraints:
     Regex:
       # @see https://getcomposer.org/schema.json
-      pattern: '/^[a-z0-9]([_.-]?[a-z0-9]+)*/[a-z0-9](([_.]|-{1,2})?[a-z0-9]+)*$/'
+      pattern: '/^[a-z0-9]([_.-]?[a-z0-9]+)*\/[a-z0-9](([_.]|-{1,2})?[a-z0-9]+)*$/'
 
 package_manager.settings:
   type: config_object