Multi-module projects

This proposal is to have projects that consist of more than one module, where the main module imports others. The UI, I suppose, would look like tabs within the editor area. The goal would be to keep this as simple as possible for users who don't want it. I am not interested in making the UI more complex for beginners in order to support a few advanced students; but it would be good to have a reasonable path for those advanced students.

This also makes Haskell-mode much more appealing as a general-purpose tool for learning Haskell.

The issue names no files, tests, or entry points. Begin by mapping how the editor currently handles a single module and how the UI could represent imported modules without adding complexity for beginners. Done would mean a main module can import additional modules and advanced users have a clear multi-module workflow.

Written by the indexing model from the issue text.