#3075696 Search index related functions have been moved to a service

from:
  fqn: 'fn:search_index'
to:
  fqn: '\Drupal\search\SearchIndexInterface::index'
from:
  fqn: 'fn:search_index_clear'
to:
  fqn: '\Drupal\search\SearchIndexInterface::clear'
from:
  fqn: 'fn:search_mark_for_reindex'
to:
  fqn: '\Drupal\search\SearchIndexInterface::markForReindex'
from:
  fqn: 'fn:search_update_totals'
to:
  fqn: '\Drupal\search\SearchIndexInterface::updateWordWeights'
to:
  fqn: \Drupal\search\SearchIndexInterface