Drush says it needs Composer, but Composer is already installed? I've recently re-provisioned my development box, and went to run a drush command but was met with


  Unable to load autoload.php. Drush now requires Composer in order to install its dependencies and autoload classes. Please see README.md


I then tested to see if Composer was installed and found it was (version 1.0.3).

Would anyone know what I can do here?
There is a Drupal core isssue about this: Catch error(s) from composer install not being run and return friendly error.

tl;dr

Basically, you need to run composer install from inside your docroot.