Out of date documentation for Dev

[Eh2406]

In #108 an important piece of documentation was missed. The "Package and Version traits" section rendered and code is wrong and out of date. This needs to be updated to describe the VersionSet trait.

[Eh2406]

Also here https://github.com/pubgrub-rs/pubgrub/blob/dev/src/solver.rs#L28

[Eh2406]

https://github.com/pubgrub-rs/pubgrub/blob/dev/src/lib.rs#L22 and https://github.com/pubgrub-rs/pubgrub/blob/dev/src/solver.rs#L28 are still wrong on dev.

[Eh2406]

One of the reasons this was missed in my review is that https://github.com/pubgrub-rs/pubgrub/blob/dev/src/solver.rs#L28 is module documentation for a now private module. That documentation should get moved to resolve.