Revert "top-level: Deprecate top-level `{build,host,target}Platform`"

   <para>

    In Nixpkgs, these three platforms are defined as attribute sets under the

    names <literal>buildPlatform</literal>, <literal>hostPlatform</literal>, and

    <literal>targetPlatform</literal>. They are always defined as attributes in

    the standard environment. That means one can access them like:

    names <literal>buildPlatform</literal>, <literal>hostPlatform</literal>,

    and <literal>targetPlatform</literal>. All three are always defined as

    attributes in the standard environment, and at the top level. That means

    one can get at them just like a dependency in a function that is imported

    with <literal>callPackage</literal>:

<programlisting>{ stdenv, buildPlatform, hostPlatform, fooDep, barDep, .. }: ...buildPlatform...</programlisting>

    , or just off <varname>stdenv</varname>:

<programlisting>{ stdenv, fooDep, barDep, .. }: ...stdenv.buildPlatform...</programlisting>

    .

   </para>

  # The old identifiers for cross-compiling. These should eventually be removed,

  # and the packages that rely on them refactored accordingly.

  platformCompat = self: super: {

    buildPlatform = lib.warn

      "top-level `buildPlatform` is deprecated since 18.09. Please use `stdenv.buildPlatform`."

      super.stdenv.buildPlatform;

    hostPlatform = lib.warn

      "top-level `hostPlatform` is deprecated since 18.09. Please use `stdenv.hostPlatform`."

      super.stdenv.hostPlatform;

    targetPlatform = lib.warn

      "top-level `targetPlatform` is deprecated since 18.09. Please use `stdenv.targetPlatform`."

      super.stdenv.targetPlatform;

    inherit (super.stdenv.hostPlatform) system;

  platformCompat = self: super: let

    inherit (super.stdenv) buildPlatform hostPlatform targetPlatform;

  in {

    inherit buildPlatform hostPlatform targetPlatform;

    inherit (buildPlatform) system;

  };

  splice = self: super: import ./splice.nix lib self (buildPackages != null);