Rename `ShortestPaths` to `SingleSourceShortestPaths`

[pnevyk]

This will follow the convention in literature and will make it easier to distinguish between single-source shortest paths and all-pairs shortest paths that we will implement in the future. The naming is present also in the Wikipedia article.

[pnevyk]

After renaming, I realized that the name is quite long, especially for such a common operation as finding a shortest path between two vertices is. Therefore I think it's not worthy and it's fine to use the generic, potentially ambiguous name, while using AllPairsShortestPaths name for the less common operation.