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/python.section.md

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