drupal/core/modules/migrate_drupal/migrate_drupal.module

<?php

/**
 * @file
 * Provides migration from other Drupal sites.
 */

use Drupal\Core\Database\DatabaseExceptionWrapper;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\migrate\Exception\RequirementsException;
use Drupal\migrate\MigrateExecutable;
use Drupal\migrate\MigrateMessage;
use Drupal\migrate\Plugin\RequirementsInterface;

/**
 * Implements hook_help().
 */
function migrate_drupal_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.page.migrate_drupal':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Migrate Drupal module provides a framework based on the <a href=":migrate">Migrate module</a> to facilitate migration from a Drupal (6, 7, or 8) site to your website. It does not provide a user interface. For more information, see the <a href=":migrate_drupal">online documentation for the Migrate Drupal module</a>.', [':migrate' => \Drupal::url('help.page', ['name' => 'migrate']), ':migrate_drupal' => 'https://www.drupal.org/documentation/modules/migrate_drupal']) . '</p>';
      return $output;
  }
}

/**
 * Implements hook_migration_plugins_alter().
 */
function migrate_drupal_migration_plugins_alter(&$definitions) {
  // This is why the deriver can't do this: the 'd6_taxonomy_vocabulary'
  // definition is not available to the deriver as it is running inside
  // getDefinitions().
  if (isset($definitions['d6_taxonomy_vocabulary'])) {
    $vocabulary_migration_definition = [
      'source' => [
        'ignore_map' => TRUE,
        'plugin' => 'd6_taxonomy_vocabulary',
      ],
      'destination' => [
        'plugin' => 'null',
      ],
    ];
    $vocabulary_migration = \Drupal::service('plugin.manager.migration')->createStubMigration($vocabulary_migration_definition);

    try {
      $source_plugin = $vocabulary_migration->getSourcePlugin();
      if ($source_plugin instanceof RequirementsInterface) {
        $source_plugin->checkRequirements();
      }
      $executable = new MigrateExecutable($vocabulary_migration, new MigrateMessage());
      $process = ['vid' => $definitions['d6_taxonomy_vocabulary']['process']['vid']];
      foreach ($source_plugin as $row) {
        $executable->processRow($row, $process);
        $source_vid = $row->getSourceProperty('vid');
        $plugin_ids = ['d6_term_node:' . $source_vid, 'd6_term_node_revision:' . $source_vid];
        foreach ($plugin_ids as $plugin_id) {
          if (isset($definitions[$plugin_id])) {
            $definitions[$plugin_id]['process'][$row->getDestinationProperty('vid')] = 'tid';
          }
        }
      }
    }
    catch (RequirementsException $e) {
      // This code currently runs whenever the definitions are being loaded and
      // if you have a Drupal 7 source site then the requirements will not be
      // met for the d6_taxonomy_vocabulary migration.
    }
    catch (DatabaseExceptionWrapper $e) {
      // When the definitions are loaded it is possible the tables will not
      // exist.
    }

  }
}