Again, accept Interfaces rather than the noPrelude flag so that we don't have to reproduce the same logic for getting the Prelude.interfaces in Compiler.hs (ie either getting the list of interfaces or an empty Map). Note that this conditional is pretty much identical to the one that is [already in Compiler.hs](https://github.com/evancz/Elm/pull/338/files#diff-ce216d87602264cb457bbef16ffe9a9eR187) so it's nice to not have to reproduce this logic in case it changes later.