MDL-71031 external: Minor tweaks towards coding style and external rules

- Some small namespace/use/MOODLE_INTERNAL/requires reorganization.
- Move file phpdocs to class phpdocs.
- Move external method names to be new "execute" default ones.
- Move unit tests to matching core_grades\external namespace.
- Add missing clean_returnvalue() calls.

lib/db/services.php

@@ -942,7 +942,6 @@ $functions = array(
     ),
     'core_grades_create_gradecategories' => array (
         'classname' => 'core_grades\external\create_gradecategories',
-        'methodname' => 'create_gradecategories',
         'description' => 'Create grade categories inside a course gradebook.',
         'type' => 'write',
         'capabilities' => 'moodle/grade:manage',