Conclusion

My conclusion would be to make module.inc the key module for module and project dependency management.

Changes to install.php, system.module, update.php are relative easy to do afaik.

What i haven't covered is schema update dependencies.

Steps to take:

  1. make drush_mm more robust as a showcase
  2. patch module.inc with lessons learned from drush_mm
  3. provide the project provides files same like status_update
  4. with this into place drush (D7) could have lean and mean drush_pm and drush_mm modules
  5. project version dependency management is maybe a GSOC project?