Related to dotnet/designs#181

Regardless of how we ultimately decide to ship inbox source-generators / analyzers we will need to be able to conflict resolve between inbox components and those delivered in NuGet packages. This should happen after items are raised from nuget packages, after frameworks are resolved (in case we do add support for framework-specific generators/analyzers), after any SDK adds them, and before compile.

This is needed for .NET 6.0 to accommodate components adding source generators which ship in both nuget packages and inbox.

cc @dsplaisted