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/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`.