What is the equivalent of calling drupal_lookup_path()?
Drupal 7: 

$path .= drupal_lookup_path('alias', $path);


Drupal 8:

$alias = 
  \Drupal::service('path.alias_manager')
    -&gt;getPathAlias($path);

if (strcmp($alias, $path) == 0) {
  $alias = '';  // No alias was found.
}

$path .= $alias;


Reference Link

8 calls to drupal_lookup_path()


drupal_get_normal_path
drupal_get_path_alias
purl_get_normal_path
rules_condition_path_alias_exists
rules_condition_path_has_alias