lib: make extensible

This allows the lib fixed point to be extended with

  myLib = lib.extend (self: super: {
    foo = "foo";
  })

With this it's possible to have the new modified lib attrset available to all
modules when using evalModules

  myLib.evalModules {
    modules = [ ({ lib, ... }: {
      options.bar = lib.mkOption {
	default = lib.foo;
      };
    }) ];
  }

  => { config = { bar = "foo"; ... }; options = ...; }

lib/default.nix

@@ -5,9 +5,11 @@
  */
 let
 
-  callLibs = file: import file { inherit lib; };
+  inherit (import ./fixed-points.nix {}) makeExtensible;
 
-  lib = rec {
+  lib = makeExtensible (self: let
+    callLibs = file: import file { lib = self; };
+  in with self; {
 
     # often used, or depending on very little
     trivial = callLibs ./trivial.nix;
@@ -128,5 +130,5 @@ let
       mergeAttrsNoOverride mergeAttrByFunc mergeAttrsByFuncDefaults
       mergeAttrsByFuncDefaultsClean mergeAttrBy
       prepareDerivationArgs nixType imap overridableDelayableArgs;
-  };
+  });
 in lib

lib/modules.nix

@@ -59,7 +59,7 @@ rec {
         };
       };
 
-      closed = closeModules (modules ++ [ internalModule ]) ({ inherit config options; lib = import ./.; } // specialArgs);
+      closed = closeModules (modules ++ [ internalModule ]) ({ inherit config options lib; } // specialArgs);
 
       options = mergeModules prefix (reverseList (filterModules (specialArgs.modulesPath or "") closed));