Add devShell submodule; allow disabling dev environment

[srid]

Resolves #37 (to be squash merged)

• Group buildTools (renamed to tools), hlsCheck and hlintCheck under the devShell submodule option
• Allow disabling them all using devShell.enable = false; (useful if you want haskell-flake to produce just the package outputs).

cc @Kranzes

[srid]

Aside: regarding defaultBuildTools, perhaps that can be decoupled into a separate haskell-flake module, having the user explicitly do:

haskellProjects.foo.imports = [
  inputs.haskell-flake.defaults
];

Less magic this way, as suggested by @maralorn

[maralorn]

Nice. I didn’t know that there was a trick to null some of those default tools. Having some tools in there by default is fine. You shouldn’t change that.

I would just suggest that you list which tools those are in the example and in the option description. Also explain how to null them in the option description, probably best via the example value for the option.

[Kranzes]

Good stuff Srid, I remember I suggested you to have it this way months ago and you said that it defeats the whole purpose of the project which I disagreed with. It's good that now I won't be forced to having a devshell when sometimes all I want is to package something with a better interface for cabal2nix.