Support `nix flake show --option allow-import-from-derivation false`

This will allow inclusion of haskell-flake based projects into the NixOS flakes search.

I believe it can be done, but we'll have to make the set of attributes less dynamic, in particular we have to predict outputs, the names of the outputs.

lib.lazyDerivation https://github.com/NixOS/nixpkgs/commit/fce8b018f06431e7684b725a520416ff3862db9f produces an attrset that is a valid "derivation attrset" but only needs to evaluate the argument derivation for attributes that really need it, like drvPath, outPath, etc.

Our package definitions would have to look somewhat like

fix (pkg:
  lazyDerivation {
    derivation = callCabal2nix ...;
    meta = ... hardcoded or perhaps some `readFile` on `.cabal` sources if we dare ...;
    passthru = {
      inherit (pkg)
        overrideCabal
        overrideAttrs
        # ... any useful unconditional attributes
        ;
    };
  }
)

I can add multi-output support to lazyDerivation.

Start by examining the flake outputs and package definitions around callCabal2nix, then review nixpkgs' lib.lazyDerivation reference linked in the issue. Determine the output names and unconditional attributes that must remain available without import-from-derivation. Done means the flake can be shown with allow-import-from-derivation set to false, including multi-output packages.