Refactored VCS dependencies a bit

[apojomovsky]

Description

Renamed .repos files to reflect reality. Here, the dependencies.repos was pointing to the aws house, which is not even a real dependency of the repository. On the other hand, the repo targetted by msgs.repos is an absolutely vital dependency for the sim to work. So I renamed the files this way:

• dependencies.repos -> demo.repos (open to suggestions)
• msgs.repos -> dependencies.repos (not that open to suggestions, but still open)

Documentation as well as CI scripts and gitignore was updated to reflect these changes. Also, the irobot_create_msgs URI was updated from HTTPS to SSH. As this is a private repo, it's a pain to use login credentials on every push/pull from and to that repo.

Checklist

• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation

[apojomovsky]

Comments addressed @alsora / @eborghi10 . PTAL