Deprecate magic ServiceProvider file discovery; declare in services.yml

Closes #2910814

core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php

@@ -476,8 +476,8 @@ private function validate($content, $file)
             throw new InvalidArgumentException(sprintf('The service file "%s" is not valid. It should contain an array. Check your YAML syntax.', $file));
         }
 
-        if ($invalid_keys = array_keys(array_diff_key($content, ['parameters' => 1, 'services' => 1]))) {
-            throw new InvalidArgumentException(sprintf('The service file "%s" is not valid: it contains invalid root key(s) "%s". Services have to be added under "services" and Parameters under "parameters".', $file, implode('", "', $invalid_keys)));
+        if ($invalid_keys = array_keys(array_diff_key($content, ['parameters' => 1, 'services' => 1, 'service_providers' => 1]))) {
+            throw new InvalidArgumentException(sprintf('The service file "%s" is not valid: it contains invalid root key(s) "%s". Services have to be added under "services", Parameters under "parameters", and service providers under "service_providers".', $file, implode('", "', $invalid_keys)));
         }
 
         return $content;