RFC: *: move away from rules_nixpkgs

This is an attempt to see how well we do without rules_nixpkgs.

rules_nixpkgs has the following problems:

 - complicates our build system significantly (generated external
   repository indirection for picking local/nix python and go)
 - creates builds that cannot run on production (as they are tainted by
   /nix/store libraries)
 - is not a full solution to the bazel hermeticity problem anyway, and
   we'll have to tackle that some other way (eg. by introducing proper
   C++ cross-compilation toolchains and building everything from C,
   including Python and Go)

Instead of rules_nixpkgs, we ship a shell.nix file, so NixOS users can
just:

  jane@hacker:~/hscloud $ nix-shell
  hscloud-build-chrootenv:jane@hacker:~/hscloud$ prodaccess

This shell.nix is in a way nicer, as it immediately gives you all tools
needed to access production straight away.

Change-Id: Ieceb5ae0fb4d32e87301e5c99416379cedc900c5
diff --git a/WORKSPACE b/WORKSPACE
index f576986..95cecf9 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -25,21 +25,6 @@
     sha256 = "e46612e9bb0dae8745de6a0643be69e8665a03f63163ac6610c210e80d14c3e4",
 )
 
-# Load and setup Nixpkgs, if Nix is present on the build system.
-http_archive(
-    name = "io_tweag_rules_nixpkgs",
-    strip_prefix = "rules_nixpkgs-dc24090573d74adcf38730422941fd69b87682c7",
-    urls = ["https://github.com/tweag/rules_nixpkgs/archive/dc24090573d74adcf38730422941fd69b87682c7.tar.gz"],
-    sha256 = "aca86baa64174478c57f74ed09d5c2313113abe94aa3af030486d1b14032d3ed",
-)
-
-load("//third_party/nix:repository_rules.bzl", "hscloud_setup_nix")
-
-hscloud_setup_nix(
-    revision = "1179841f9a88b8a548f4b11d1a03aa25a790c379",
-    sha256 = "8b64041bfb9760de9e797c0a985a4830880c21732489f397e217d877edd9a990",
-)
-
 # Download Go/Gazelle rules
 http_archive(
     name = "io_bazel_rules_go",
@@ -69,9 +54,9 @@
 
 pip_repositories()
 
-load("@hscloud_pip_imports//:imports.bzl", "hscloud_pip3_import")
+load("@rules_python//python:pip.bzl", "pip3_import")
 
-hscloud_pip3_import(
+pip3_import(
     name = "pydeps",
     requirements = "//third_party/py:requirements.txt",
 )
@@ -81,12 +66,8 @@
 pip_install()
 
 # Setup Go toolchain.
-# This workspace is generated by hscloud_setup_nixpkgs. It will either call
-# go_register_toolchains() to automagically get Go toolchains from the Internet
-# or, if nix is present, instead setup a toolchain from nixpkgs.
-load("@hscloud_go_toolchain//:imports.bzl", "hscloud_go_register_toolchains")
-
-hscloud_go_register_toolchains()
+load("@io_bazel_rules_go//go:deps.bzl", "go_register_toolchains")
+go_register_toolchains()
 
 # IMPORTANT: match protobuf version above with the one loaded by grpc
 http_archive(