Fix a bug when switching Snakeoil repository from Development -> stable

Switching Snakeoil repo from stable to experimental and then back to stable will remove all Snakeoil references.

Correct procedure should be:

1. When adding experimental, create a new source for experiment
2. But keep the original stable source
3. When moving from experimental to stable, remove the experimental source

Doing it this way, in theory the user will have an update if there's a newer stable version to override the older experimental version.