How to set/get weight for modules and how to enable module programatically in druapl 8

In Druapl 7 the "system" table will have all the module enable and disable status. In druapl 8

#1608842: Replace list of bootstrap modules, enabled modules, enabled themes, and default theme with config

#1808132: Complement module_set_weight() with module_get_weight()

 

Description: 

If you were running any queries against the {system} table, these won't work any more. For example:
Drupal 7:

<?php
  // Enable the admin theme.
  db_update('system')
    ->fields(array('status' => 1))
    ->condition('type', 'theme')
    ->condition('name', 'seven')
    ->execute();
?>

Drupal 8:

<?php
  theme_enable(array('seven'));
?>

Enable Module: 

 module_enable(array($module), FALSE); 

 module_enable($module_list, $enable_dependencies = TRUE) 

Reff: http://www.drupalcontrib.org/api/drupal/drupal!core!includes!module.inc/function/module_enable/8

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.

Source : https://www.drupal.org/node/1813642