Unnecessary configuring on (almost?) any file change

[snoyberg]

I've noticed that almost any time I have a file change in a project, stack build will run the Cabal configure step, slowing down compilations. I've traced this down to some issue with stack-setup-config-mod, and it appears that the file we're checking for dirtiness does not exist, triggering regular rebuilds.

I did a blame, and I think this comes down to this commit:

681ca33

It looks to me like stack-setup-config-mod is never being created in this commit. If I'm not mistaken, we need to add an equivalent to the writeCabalMod call that appears in this module.

CC @qrilka, I can take a stab at a PR, but input on this would be appreciated.

[qrilka]

I do remember this and I thought that fixed this in some follow-up PR but it looks that the fix didn't land on github somehow, that's odd...