hash all the files

[domenkozar]

👋

I've ported nix-direnv to devenv.sh here: cachix/devenv#387

The motivation is to simplify the whole script, but to do a few devenv related things like watching all the imported files.

But I've noticed that currently only flake.nix is hashed, while flake.lock should also be hashed - see the source of my implementation.