Make php generated code conform to PSR-4. (#2435)

1. Generate single file for each message.
2. Lazily initiate metadata.

php/tests/generated_class_test.php

diff --git a/php/tests/generated_class_test.php b/php/tests/generated_class_test.php
index b3ecd3a..27912ec 100644
--- a/php/tests/generated_class_test.php
+++ b/php/tests/generated_class_test.php
@@ -1,7 +1,6 @@
 <?php
 
-require_once('test.pb.php');
-require_once('test_no_namespace.pb.php');
+require_once('generated/NoNameSpace.php');
 require_once('test_util.php');
 
 use Google\Protobuf\Internal\RepeatedField;