Refactor `drush convert-field-collection` into callable functions for update hooks

I am having issues running the `drush convert-field-collection` in an update hook (http://drupal.stackexchange.com/questions/206662/how-to-boostrap-drush-f...). It seems it would be easier if the function was simply a wrapper around the functionality that we could then call from within an update hook.

I am looking for feedback on if this is the correct approach or maybe I am missing something obvious. If this is correct I can work on a patch.