Provide a default get_preference() implementation

[uranusjr]

Cargo sorts the resolution order by number of available candidates (resolve package with fewer candidates first). This matches my (instinctive) choice in Passa, and could actually be the answer?

https://github.com/rust-lang/cargo/blob/4fbd644bc76e25815a8e3a2189b6430fe69fd0d6/src/cargo/core/resolver/types.rs#L205-L219

https://github.com/sarugaku/passa/blob/2ac00f16cd5a8f07d679a3ab02b7cc13c6f42bee/src/passa/models/providers.py#L44-L47

But I still think more real-world cases need to be tested before we can settle on a good default. (Note that the default implementation should be somewhat cheap, since this is called on every resolution round.)

[pradyunsg]

Using the number of available candidates for a requirement, to determine the order of exploration, sounds like a reasonable default to me.