Improve gathering of copy items

1. In #8720 we improved the gathering of copyable items considerably by handling the `ReferenceOutputAssembly`, `Private` and `EmbedInteropTypes` metadata in a simple way. There is more complexity to these rules however, and we should update the code accordingly. This work should also document those rules and include unit tests.
2. Review the [outputs of the RAR](https://github.com/dotnet/msbuild/blob/100e592026e3fbcdb273e9d8349daad377b40886/src/Tasks/Microsoft.Common.CurrentVersion.targets#L2398-L2410) task to see whether we need to add other kinds of items too, such as satellite assemblies, scatter files, related files, etc.