Revert "shellFor: Don't suck in src to compare to deps. [Fixes #51079]"

This reverts commit b40b39b.

tarball job failed:

    error: while evaluating anonymous function at /build/source/maintainers/scripts/find-tarballs.nix:6:1, called from undefined position:
    while evaluating 'operator' at /build/source/maintainers/scripts/find-tarballs.nix:27:16, called from undefined position:
    while evaluating 'immediateDependenciesOf' at /build/source/maintainers/scripts/find-tarballs.nix:39:29, called from /build/source/maintainers/scripts/find-tarballs.nix:27:44:
    while evaluating anonymous function at /build/source/lib/attrsets.nix:225:10, called from undefined position:
    while evaluating anonymous function at /build/source/maintainers/scripts/find-tarballs.nix:40:37, called from /build/source/lib/attrsets.nix:225:16:
    while evaluating 'derivationsIn' at /build/source/maintainers/scripts/find-tarballs.nix:42:19, called from /build/source/maintainers/scripts/find-tarballs.nix:40:40:
    while evaluating 'optional' at /build/source/lib/lists.nix:241:20, called from /build/source/maintainers/scripts/find-tarballs.nix:44:33:
    while evaluating 'canEval' at /build/source/maintainers/scripts/find-tarballs.nix:48:13, called from /build/source/maintainers/scripts/find-tarballs.nix:44:43:
    while evaluating the attribute 'drvPath' at /build/source/lib/customisation.nix:149:7:
    while evaluating the derivation attribute 'name' at /build/source/pkgs/stdenv/generic/make-derivation.nix:177:11:
    while evaluating 'nullSrc' at /build/source/pkgs/development/haskell-modules/make-package-set.nix:263:19, called from undefined position:
    while evaluating 'overrideCabal' at /build/source/pkgs/development/haskell-modules/lib.nix:37:24, called from /build/source/pkgs/development/haskell-modules/make-package-set.nix:263:22:
    attribute 'override' missing, at /build/source/pkgs/development/haskell-modules/lib.nix:37:28

pkgs/development/haskell-modules/make-package-set.nix

@@ -260,10 +260,7 @@ in package-set { inherit pkgs stdenv callPackage; } self // {
     #     bash$ nix-shell --run "cabal new-build all"
     shellFor = { packages, withHoogle ? false, ... } @ args:
       let
-        nullSrc = p: overrideCabal p (_: { src = null; });
-
-        # Make sure we *never* accidentally suck in src.
-        selected = map nullSrc (packages self);
+        selected = packages self;
 
         packageInputs = map getBuildInputs selected;
 
@@ -275,8 +272,7 @@ in package-set { inherit pkgs stdenv callPackage; } self // {
         # because cabal will end up ignoring that built version, assuming
         # new-style commands.
         haskellInputs = pkgs.lib.filter
-          # nullSrc in case a dep is one of the selected packages.
-          (input: pkgs.lib.all (p: (nullSrc input).outPath != p.outPath) selected)
+          (input: pkgs.lib.all (p: input.outPath != p.outPath) selected)
           (pkgs.lib.concatMap (p: p.haskellBuildInputs) packageInputs);
         systemInputs = pkgs.lib.concatMap (p: p.systemBuildInputs) packageInputs;