Language-related modules not listed on available updates page

Problem/motivation

Due to the new Multilingual module group introduced in #1833184: Find a consistent naming scheme for translation-related modules, update module does not treat language, content translation and interface translation modules as part of core. This does not regress any functionality, since the modules are grouped to be under project "" (empty string), and ignored for update module. Since there are other core modules, core itself is checked for updates. The only visible resulting problem is these modules are not listed anymore as part of core on the available updates page:

Proposed solution

Do not use the 'Core' package as sole indication of a module being in core. The fix in #1871328: Modules in new Multilingual package are no longer tested by Module\EnableDisableTest for example used new conditions to check for the path. The problematic code is:

function update_get_project_name($file) {
  $project_name = '';
  if (isset($file->info['project'])) {
    $project_name = $file->info['project'];
  }
  elseif (isset($file->info['package']) && (strpos($file->info['package'], 'Core') === 0)) {
    $project_name = 'drupal';
  }
  return $project_name;
}

Also would need a small extension to tests to enable Language module and check it shows up on the available updates page (or if we want to test the pure functionality, to check if language module is part of the core project as decided by update_get_project_name().