Fix php well known type conformance tests (#3828)

* Fix php well known type conformance tests

* Properly generate code for test.proto

* Provide GPBMetadata files in c extensions for generated files to import.

* Remove unnecessary test

* Clean up code

* Add declaration for initOnce.

* Refactoring
diff --git a/conformance/autoload.php b/conformance/autoload.php
index 2cee31c..0f49aec 100644
--- a/conformance/autoload.php
+++ b/conformance/autoload.php
@@ -2,7 +2,7 @@
 
 define("GOOGLE_INTERNAL_NAMESPACE", "Google\\Protobuf\\Internal\\");
 define("GOOGLE_NAMESPACE", "Google\\Protobuf\\");
-define("GOOGLE_GPBMETADATA_NAMESPACE", "GPBMetadata\\Google\\Protobuf\\Internal\\");
+define("GOOGLE_GPBMETADATA_NAMESPACE", "GPBMetadata\\Google\\Protobuf\\");
 
 function protobuf_autoloader_impl($class, $prefix) {
     $length = strlen($prefix);