Add `inputMap` to `stackProject`

[hamishmack]

trafficstars

It might be sensible to add the inputMap feature from cabalPorject to stackProject (see #374).

It works a lot like sha256map (but providing the actual input rather than just the hash) and sha256map is supported in stackProject and it is used in lib/stack-cache-generator.nix. Most likely the way to add inputMap support is to update that code.

[thomasjm]

I'd really like to get this working, since it seems to be the only way to get private git repositories to work with stackProject. (Unless I'm mistaken about that?)

I can take a stab at doing this, but could you point me in the right direction?