Review resolution ordering

[uranusjr]

Slightly related to #11.

Currently our resolver picks one package to resolve for each iteration by looking all known requirements. In contrast, Rust uses a DFS, i.e. it picks one initial requirement to resolve, and pick one of its dependency to resolve, and so on, completing the whole branch before looking at the next initial requirement. My instinct was that looking at all known requirements (so more of a BFS) makes it easier to detect conflicts early, but that might not be the case.

Again, real-world cases would help greatly to determine which choice is better.

[uranusjr]

Interestingly Molinilo takes the complete opposite strategy: The requirement with the most candidates (with the loosest constraint) is visited first!