Update theme handling to D8.
Update cache-rebuild to align with latest code in core/rebuild.php. Also reduce bootstrap level to DRUSH_BOOTSTRAP_DRUPAL_ROOT.
Better checks for drupal bootstrap status. Fix Unish\pmDownloadCase::testSelect.
Revert "Better checks for drupal bootstrap status. Fix Unish\pmDownloadCase::testSelect."

This reverts commit d0ae2496f315058946edbfbb0e096024bc57f25c.

/me messed up with branches.
Follow-up to d809c2d58: There's no need to include the drupal environment engine.
Add an outline of a new 'drush' engine for update_info.
update_info drush engine: get releases info and manage caches.
update_info drush engine: discard caches if ctime of project is newer than cache.
update_info drush engine: remove download cache of update files.
Refactoring of release_info's updatexml engine.
Set 'drush' as default update_info engine.
Fix releaseInfo tests.
Fix reference to renamed function.
Fix wrong method name.
Better check for drupal bootstrap status. Fix Unish\pmDownloadCase::testSelect