Run scripts even when Packages folder doesn't exist

I tried running a simple Hello, World script and got an exception because there was no packages folder in the current directory. I don't think you need this folder if you're not using any nuget packages, so I just return an empty list of paths.

I also got an error because IFileSystem was not found in the container, it seemed that the right fix was to add the ScriptCS.Core assembly to the MEF configuration.