The new DrupalExistingComponentTask class exposes a task that can
search through a directory and return all of the module packages
in the directory. It can optionally find only the ones that have
new releases not yet installed.
Most of the code was written by the time I got around to creating
a git repository. Originally, this was intended to go into the
Drupal CVS repository, but there are several reasons why this
code cannot be added there.