can't JOIN to a foreign database whose name contains a hyphen

If a database name contains a hyphen, which is allowable with MySQL, then joining to it as a foreign database table will crash.

This comes up when using Migrate, if the Drupal 8 site's database name contains hyphens, as SqlBase::initializeIterator() does a join back to the D8 site's database as a foreign database.

Sample code:

  $query = \Drupal::database()->select('node_field_data', 'nfd');
  $query->addField('nfd', 'title');
  $alias = $query->leftJoin('my-other-database.node', 'foreign_node', "nfd.nid = foreign_node.nid")
  $query->range(0, 1);
  $nid = $query->execute()->fetchField();

The problem is that Connection::escapeTable() is too strict. It needs to allow the hyphen, and also the backtick, so that the following can be done:

  $alias = $query->leftJoin('`my-other-database`.node', 'foreign_node', "nfd.nid = foreign_node.nid")