Do not always include Prelude import into IL

@fte @mh This should minimize the problem we talked about. Previously, the Prelude was unconditionally included in the list of imported Modules after the transformation to the intermediate language. I've removed that import. To ensure that the prelude is still imported whenever a module needs it (due to an (implicit) import, or some frontend code transformations), I've extended the list of computed imports to also include all modules required in type annotations. This ensures that this change does not lead to accidentally un-closed flat curry modules. The latter should also fix a Problem I've had with the KMCC and mentioned to @fte.