Set Module priority

Setting module weights:
Drupal 7:

<?php
  // Set a module's weight.
  db_update('system')
    ->fields(array('weight' => $weight))
    ->condition('name', $mymodule)
    ->execute();
?>

Drupal 8:

<?php
  module_set_weight($mymodule, $weight);
?>

To set a module's weight relative to another, a module_get_weight function is planned.