fix Drush command sql-query with option "--db-prefix" in use (#3529)

* fix namespace for Drush command sql-query with option "--db-prefix" in use

* [#2882] make option "--db-prefix" work with all major versions of Drupal

* [#2882] store function call result to a variable to avoid multiple function calls

lib/Drush/Sql/SqlBase.php

@@ -2,7 +2,6 @@
 
 namespace Drush\Sql;
 
-use Drupal\Core\Database\Database;
 use Drush\Log\LogLevel;
 use Webmozart\PathUtil\Path;
 
@@ -201,8 +200,12 @@ public function query_prefix($query) {
     if (drush_has_boostrapped(DRUSH_BOOTSTRAP_DRUPAL_DATABASE)) {
       // Enable prefix processing which can be dangerous so off by default. See http://drupal.org/node/1219850.
       if (drush_get_option('db-prefix')) {
-        if (drush_drupal_major_version() >= 7) {
-          $query = Database::getConnection()->prefixTables($query);
+        $drupal_major_version = drush_drupal_major_version();
+        if ($drupal_major_version >= 8) {
+          $query = \Drupal\Core\Database\Database::getConnection()->prefixTables($query);
+        }
+        elseif ($drupal_major_version == 7) {
+          $query = \Database::getConnection()->prefixTables($query);
         }
         else {
           $query = db_prefix_tables($query);