refactor: remove Pool from API

[baszalmstra]

The Pool object implements interning but the solver doesn't care about that at all. It just needs to be able to refer to IDs and be able to call functions on the DependencyProvider using those IDs. This PR refactors the code to remove any reference of the Pool from the Solver. Instead, additional methods are added to the DependencyProvider to convert the IDs (which the DependencyProvider already provides), to strings and to filter a list of solvables with a VersionSetId (previously implemented in the VersionSet trait). The DependencyProvider trait now encapsulates everything required to implement a packaging ecosystem without requiring the use of a Pool. 📦 🍿

This should make it easier to create bindings to other languages because the actual datatypes used don't even need to be stored inside resolvo itself. They can be kept completely externally as long as the code can provide the proper Ids.

I was also able to remove loads of trait bounds through this refactor!

The Pool is still a useful tool for interning purposes so I kept it. Both the tests and the implementation in rattler most likely still benefit from it.

This is a pretty significant refactor, which I would be happy to explain offline. 😄

TODO:

• Use the new API in rattler to see if anything is missing.