mod/resource: MDL-18691 allow custom resource types to specify their name in their...

mod/resource: MDL-18691 allow custom resource types to specify their name in their own lang directory

Before this change, the full name of the resource type had to be stored in the "resource" namespace (for example in /mod/resource/lang/en_utf8/resource.php).

Now it can be put in the "resource_foo" namespace:

e.g. in /mod/resource/type/foo/lang/en_utf8/resource_foo.php:

<?php
  $string['resourcetypefoo'] = 'Link to a Foo Bar document';
?>