mkShell: introduce packages argument (#122180)

The distinction between the inputs doesn't really make sense in the
mkShell context.  Technically speaking, we should be using the
nativeBuildInputs most of the time.

So in order to make this function more beginner-friendly, add "packages"
as an attribute, that maps to nativeBuildInputs.

This commit also updates all the uses in nixpkgs.

pkgs/build-support/agda/default.nix

@@ -1,6 +1,6 @@
 # Builder for Agda packages.
 
-{ stdenv, lib, self, Agda, runCommandNoCC, makeWrapper, writeText, mkShell, ghcWithPackages, nixosTests }:
+{ stdenv, lib, self, Agda, runCommandNoCC, makeWrapper, writeText, ghcWithPackages, nixosTests }:
 
 with lib.strings;
 

pkgs/build-support/mkshell/default.nix

@@ -3,18 +3,22 @@
 # A special kind of derivation that is only meant to be consumed by the
 # nix-shell.
 {
-  inputsFrom ? [], # a list of derivations whose inputs will be made available to the environment
-  buildInputs ? [],
-  nativeBuildInputs ? [],
-  propagatedBuildInputs ? [],
-  propagatedNativeBuildInputs ? [],
-  ...
+  # a list of packages to add to the shell environment
+  packages ? [ ]
+, # propagate all the inputs from the given derivations
+  inputsFrom ? [ ]
+, buildInputs ? [ ]
+, nativeBuildInputs ? [ ]
+, propagatedBuildInputs ? [ ]
+, propagatedNativeBuildInputs ? [ ]
+, ...
 }@attrs:
 let
   mergeInputs = name: lib.concatLists (lib.catAttrs name
-    ([attrs] ++ inputsFrom));
+    ([ attrs ] ++ inputsFrom));
 
   rest = builtins.removeAttrs attrs [
+    "packages"
     "inputsFrom"
     "buildInputs"
     "nativeBuildInputs"
@@ -26,15 +30,15 @@ in
 
 stdenv.mkDerivation ({
   name = "nix-shell";
-  phases = ["nobuildPhase"];
+  phases = [ "nobuildPhase" ];
 
   buildInputs = mergeInputs "buildInputs";
-  nativeBuildInputs = mergeInputs "nativeBuildInputs";
+  nativeBuildInputs = packages ++ (mergeInputs "nativeBuildInputs");
   propagatedBuildInputs = mergeInputs "propagatedBuildInputs";
   propagatedNativeBuildInputs = mergeInputs "propagatedNativeBuildInputs";
 
   shellHook = lib.concatStringsSep "\n" (lib.catAttrs "shellHook"
-    (lib.reverseList inputsFrom ++ [attrs]));
+    (lib.reverseList inputsFrom ++ [ attrs ]));
 
   nobuildPhase = ''
     echo