Follows proper autoloading standards (#3123)

* Follows proper autoloading standards
 - Splits PHP classes in descriptor.php into separate files
 - Splits MapFieldIter and RepeatedFieldIter into separate files
 - Moves descriptor.php to Internal/functions.php
 - Moves all namespaced functions into Iternal/functions.php

* fixes Makefile.am for added php files

* [PHP] moves all functions to GPBUtil

* removes description.php from the makefile

php/composer.json

diff --git a/php/composer.json b/php/composer.json
index 32b0f44..724a45d 100644
--- a/php/composer.json
+++ b/php/composer.json
@@ -20,9 +20,6 @@
       "GPBMetadata\\": "tests/generated/GPBMetadata",
       "GPBMetadata\\Google\\Protobuf\\Internal\\": "src/GPBMetadata/Google/Protobuf/Internal",
       "": "tests/generated"
-    },
-    "files": [
-      "src/Google/Protobuf/descriptor.php"
-    ]
+    }
   }
 }