Fix stack build

[ulidtko]

Hey @jappeace

After #292, stack build on master says Stack failed to construct a build plan. — I'm fixing that by suitably declaring extra-deps (which are packages missing from Stackage), see in-source comments.

Further, lts-20 is not a valid resolver (Exception while reading snapshot from lts-20: YAML exception) — so I'm specifying a correct one. Also checking in the lockfile; it should be in git.

If you only want to support cabal build — that's fine; then drop the stack.yaml. Adding stack build to CI might be another good idea.

With this patch, stack build produces a keter executable out-of-the-box.

[jappeace]

I've no issue suppoting stack it's just hard to maintain for me on nixos :s
I'm not sure why it's so difficult for nixos peeps to support stack, every other program under the sun works