Improve goto decl

Not for merge, just for discussion and help. 
#556

In this PR, navigating to an external type (`System.String`) works. You are taken to the correct type in object browser, without error message. But I can't get members to work properly (`System.String.Copy`) 

Anyone have a clue why it doesn't work? I'm not sure what NRefactory wants here to be able to find the member. Please check the diff and help!
@7sharp9 Assembly name and location could be off, but the real fix is here: the overload of DefaultUnresolvedTypeDefinition that is used expects a full type name. There is another overload where you supply namespace and name separately. So we were supplying namespace as a full type name.