determine source directory from elm.json
For now src is just hardcoded.
this seems to be related to https://github.com/hercules-ci/elm2nix/issues/6
Anyway, I think the fact ./. is always used as an src path makes paths from elm.json match (unless ../ is used). This, as well as #6, would require producing elm.json dynamically according to nix paths.
I'm also not sure what would implication of generating elm.json be. Probably this elm.json can't be editable by users and should not overwrite the original one. Also, it should be compatible with nix-shell workflow somehow.
Overall this seems to me like nontrivial. The reason why I'm looking into this that pattern described in #6 is something we're using at work and I would like to explore the option of using elm2nix for that project.