How to get the path of a drush command?
You could make a wrapper script panopoly_starter_kit.sh, and determine the path to the script via $0.  Your script would then just run your drush command, supplying the --include variable.  See the drush script for an example of how a bash script can find the directory that the script itself is located.

UPDATE:

Another option would be to use a Drush script instead of a Drush command to do your installation.  You would lose your help output, but you could still process arguments and options.  See drush topic docs-examplescript for details.