Tool to extract external sources in stack project

[purefn]

One problem with using stackProject in haskell.nix is external sources, i.e. dependencies not in stackage or hackage, with source on GitHub. You can provide a cache set to stackProject, but that's very manual and duplicates the information in the stack files.

To avoid this, we could build an exe that uses the existing code from stack-to-nix to produce nix expressions containing the data needed for calling fetchgit and build the cache set to pass to stackProject - or stackProject could transparently build the cache itself.

[cdepillabout]

I think this was added in #72 and can probably be closed.