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.

doc/builders/special/mkshell.section.md

@@ -1,15 +1,17 @@
 # pkgs.mkShell {#sec-pkgs-mkShell}
 
-`pkgs.mkShell` is a special kind of derivation that is only useful when using it combined with `nix-shell`. It will in fact fail to instantiate when invoked with `nix-build`.
+`pkgs.mkShell` is a special kind of derivation that is only useful when using
+it combined with `nix-shell`. It will in fact fail to instantiate when invoked
+with `nix-build`.
 
 ## Usage {#sec-pkgs-mkShell-usage}
 
 ```nix
 { pkgs ? import <nixpkgs> {} }:
 pkgs.mkShell {
-  # this will make all the build inputs from hello and gnutar
-  # available to the shell environment
+  # specify which packages to add to the shell environment
+  packages = [ pkgs.gnumake ];
+  # add all the dependencies, of the given packages, to the shell environment
   inputsFrom = with pkgs; [ hello gnutar ];
-  buildInputs = [ pkgs.gnumake ];
 }
 ```

doc/languages-frameworks/dotnet.section.md

@@ -10,7 +10,7 @@ with import <nixpkgs> {};
 
 mkShell {
   name = "dotnet-env";
-  buildInputs = [
+  packages = [
     dotnet-sdk_3
   ];
 }
@@ -25,7 +25,7 @@ with import <nixpkgs> {};
 
 mkShell {
   name = "dotnet-env";
-  buildInputs = [
+  packages = [
     (with dotnetCorePackages; combinePackages [
       sdk_3_1
       sdk_3_0

doc/languages-frameworks/python.section.md

@@ -245,7 +245,7 @@ let
     ps.toolz
   ]);
 in mkShell {
-  buildInputs = [
+  packages = [
     pythonEnv
 
     black

doc/languages-frameworks/ruby.section.md

@@ -106,7 +106,7 @@ let
     name = "gems-for-some-project";
     gemdir = ./.;
   };
-in mkShell { buildInputs = [ gems gems.wrappedRuby ]; }
+in mkShell { packages = [ gems gems.wrappedRuby ]; }
 ```
 
 With this file in your directory, you can run `nix-shell` to build and use the gems. The important parts here are `bundlerEnv` and `wrappedRuby`.