Exposed LoadableAssemblies on ILibraryInformation

- Expose reliable set of assembly names that load can be called on safely.
- Expose the set of assemblies from each dependency provider
- Changed kpm pack --no-source to put loadable: false in the project.json.
This means the project won't be loadable

#339
Just a naming thing: I would argue that all assemblies are *loadable*, no? So isn't this in fact `Assemblies`? An alternative would be something like `LoadableContents` because not all *contents* are loadable, but that's an even more awful name.