Allow local non-git sources

This makes the source section of recipe.toml optional, so that one can simply create a recipe with a build recipe and a source folder and not have to worry about git at all just to get a program running in redox.

I also ran into a situation where a source directory without a git repository in it caused cookbook to reset it's own origin URL to the URL specified in the recipe for that source directory. I added a check to prevent that happening to anybody else.