Fix compatibility w Panels/Panelizer directories

When running Drush commands from within either the Panels or Panelizer directories, Drush mistakenly thinks it has found a Drupal install because both modules contain a file at "includes/common.inc". This adds a simple check to see if either "panels.module" or "panelizer.module" exist, if they do then it fails, allowing drush_locate_root() to continue recursing higher up the directory tree until it finds the correct Drupal root.