Support Solution Explorer search within framework reference assemblies

#6136 adds support for showing assemblies contained within framework references in Solution Explorer. It is not yet possible to search these however. This issue is tracking that feature.

We will need another implementation of `IDependenciesTreeSearchProvider` that can do this. The implementation will be straightforward, however it needs access to the set of framework references in the project, per target.

It feels like the natural way to obtain these would be via the `DependenciesSnapshot`, however these are not easily accessible. I'm investigating what it'll take make them so.