But ghci already contains a Haskell parser - so it should be much less work.

No excuses, but part of the reason we didn't do this is because of the structure of GHCi. There's a strong separation between the bit we call the "compiler" which translates a single module into executable code, and the "compilation manager" which handles multiple module compilation and linking. The parser lives in the compiler, and the dependency-tree generation lives in the compilation manager. Cheers, Simon