Made some changes to how DTH sees wrapped projects

- Fix up the dependencies list when returning wrapped csproj and
assemblies.
- Changed how unresolved dependencies were implemented. This allows
a specific IDependencyProvider to say a dependency is resolved, but mark
it as unresolved so that fallback does not happen.
- Remove special logic from the UnresolvedDependencyProvider and made it
a first class property on LibraryDescription.
- Mark projects as unresolved if they do not have a compatible target framework

#945