Convert libs to a fixed-point

This does break the API of being able to import any lib file and get
its libs, however I'm not sure people did this.

I made this while exploring being able to swap out docFn with a stub
in #2305, to avoid functor performance problems. I don't know if that
is going to move forward (or if it is a problem or not,) but after
doing all this work figured I'd put it up anyway :)

Two notable advantages to this approach:

1. when a lib inherits another lib's functions, it doesn't
   automatically get put in to the scope of lib
2. when a lib implements a new obscure functions, it doesn't
   automatically get put in to the scope of lib

Using the test script (later in this commit) I got the following diff
on the API:

  + diff master fixed-lib
  11764a11765,11766
  > .types.defaultFunctor
  > .types.defaultTypeMerge
  11774a11777,11778
  > .types.isOptionType
  > .types.isType
  11781a11786
  > .types.mkOptionType
  11788a11794
  > .types.setType
  11795a11802
  > .types.types

This means that this commit _adds_ to the API, however I can't find a
way to fix these last remaining discrepancies. At least none are
_removed_.

Test script (run with nix-repl in the PATH):

  #!/bin/sh

  set -eux

  repl() {
      suff=${1:-}
      echo "(import ./lib)$suff" \
          | nix-repl 2>&1
  }

  attrs_to_check() {
      repl "${1:-}" \
          | tr ';'  $'\n' \
          | grep "\.\.\." \
          | cut -d' ' -f2 \
          | sed -e "s/^/${1:-}./" \
          | sort
  }

  summ() {
      repl "${1:-}" \
          | tr ' ' $'\n' \
          | sort \
          | uniq
  }

  deep_summ() {
      suff="${1:-}"
      depth="${2:-4}"
      depth=$((depth - 1))
      summ "$suff"

      for attr in $(attrs_to_check "$suff" | grep -v "types.types"); do
          if [ $depth -eq 0 ]; then
              summ "$attr" | sed -e "s/^/$attr./"
          else
              deep_summ "$attr" "$depth" | sed -e "s/^/$attr./"
          fi
      done
  }

  (
      cd nixpkgs

      #git add .
      #git commit -m "Auto-commit, sorry" || true
      git checkout fixed-lib
      deep_summ > ../fixed-lib
      git checkout master
      deep_summ > ../master
  )

  if diff master fixed-lib; then
      echo "SHALLOW MATCH!"
  fi

  (
      cd nixpkgs
      git checkout fixed-lib
      repl .types
  )

lib/types.nix

@@ -1,15 +1,16 @@
 # Definitions related to run-time type checking.  Used in particular
 # to type-check NixOS configurations.
+{ lib }:
+with lib.lists;
+with lib.attrsets;
+with lib.options;
+with lib.trivial;
+with lib.strings;
+let
 
-with import ./lists.nix;
-with import ./attrsets.nix;
-with import ./options.nix;
-with import ./trivial.nix;
-with import ./strings.nix;
-let inherit (import ./modules.nix) mergeDefinitions filterOverrides; in
-
+  inherit (lib.modules) mergeDefinitions filterOverrides;
+  outer_types =
 rec {
-
   isType = type: x: (x._type or "") == type;
 
   setType = typeName: value: value // {
@@ -95,7 +96,6 @@ rec {
   # When adding new types don't forget to document them in
   # nixos/doc/manual/development/option-types.xml!
   types = rec {
-
     unspecified = mkOptionType {
       name = "unspecified";
     };
@@ -291,7 +291,7 @@ rec {
     submodule = opts:
       let
         opts' = toList opts;
-        inherit (import ./modules.nix) evalModules;
+        inherit (lib.modules) evalModules;
       in
       mkOptionType rec {
         name = "submodule";
@@ -395,5 +395,6 @@ rec {
     addCheck = elemType: check: elemType // { check = x: elemType.check x && check x; };
 
   };
+};
 
-}
+in outer_types // outer_types.types