Solution Explorer search can be broken by skipped dataflow updates

[drewnoakes]

NuGet/NuGet.Client#5508 reduced the number of tree updates by suppressing notifications when data is unchanged.

AssetsFileDependenciesTreeSearchProvider uses GetLatestVersionAsync to obtain the latest snapshot value, relative to the current data versions of the project. However suppression of updates meant that the latest version might not be present, and the call would hang indefinitely. This would break Solution Explorer search for customers who enabled "Search External Items", have SDK-style .NET projects in the solution, and make certain project changes that lead to so-called "version-only" updates.