Have "stack ghci" detect case where main module is also specified in the modules list

[here](https://github.com/commercialhaskell/stack/issues/1606#issuecomment-253221381), Sean Leather mentions a circumstance where `stack ghci` tries to load the same Main module twice. The issue is that `Main` is in `main-is` and also in `other-modules`. We should be able to detect this circumstance, and make it a non-issue.