Better support for conditional PackageReferences

## Summary

`PackageReference` items can be conditioned on the `TargetFramework` property. This allows MAUI projects to pull in packages that are specific to a certain platform, like iOS or Android. However, we may not have the correct APIs to allow the NuGet Package Manager UI to properly add, update, remove, or query for these kinds of packages.

This work item tracks:

- Identifying the specific issues to address
- Designing an appropriate solution
- Implementing that solution.

## Related

- [NuGet Package manager does not honor conditional nuget package references](https://developercommunity.visualstudio.com/t/NuGet-Package-manager-does-not-honor-con/10269291)
- https://github.com/NuGet/Home/issues/4681