Namespace the Nix-related things

Organist isn't _just_ a Nix framework, so we want to make sure that it
is possible to add stuff that's not strictly Nix-related to it, and have
it feel first-class.
So move most of the Nix-related stuff in its own namespace to clear up
some real estate for the rest.

examples/c-hello-world/nickel.lock.ncl

@@ -1,3 +1,3 @@
 {
-  organist = import "../../lib/nix.ncl",
+  organist = import "../../lib/organist.ncl",
 }

examples/c-hello-world/project.ncl

@@ -4,21 +4,21 @@ let organist = inputs.organist in
 {
   packages."default" = packages.hello,
   packages.hello =
-    organist.builders.NixpkgsPkg
+    organist.nix.builders.NixpkgsPkg
     & {
       name = "hello",
       version = "0.1",
       nix_drv = {
-        buildInputs.gcc = organist.lib.import_nix "nixpkgs#gcc",
-        buildInputs.coreutils = organist.lib.import_nix "nixpkgs#coreutils",
-        buildInputs.bash = organist.lib.import_nix "nixpkgs#bash",
+        buildInputs.gcc = organist.import_nix "nixpkgs#gcc",
+        buildInputs.coreutils = organist.import_nix "nixpkgs#coreutils",
+        buildInputs.bash = organist.import_nix "nixpkgs#bash",
         buildCommand =
           nix-s%"
-          gcc %{organist.lib.import_file "hello.c"} -o hello
+          gcc %{organist.nix.builtins.import_file "hello.c"} -o hello
           mkdir -p $out/bin
           cp hello $out/bin/hello
         "%
-            | organist.contracts.NixString,
+            | organist.nix.derivation.NixString,
       },
     }
 }

flake.nix

@@ -29,7 +29,7 @@
     outputsFromNickel = baseDir: flakeInputs: {
       systems ? flake-utils.lib.defaultSystems,
       lockFileContents ? {
-        organist = "${self}/lib/nix.ncl";
+        organist = "${self}/lib/organist.ncl";
       },
     }:
       flake-utils.lib.eachSystem systems (system: let
@@ -56,7 +56,7 @@
         });
 
     computedOutputs = outputsFromNickel ./. inputs {
-      lockFileContents.organist = "./lib/nix.ncl";
+      lockFileContents.organist = "./lib/organist.ncl";
     };
   in
     {

lib/lib.nix

@@ -46,15 +46,15 @@
   #     organist = {
   #       builders = "/nix/store/...-source/builders.ncl";
   #       contracts = "/nix/store/...-source/contracts.ncl";
-  #       nix = "/nix/store/...-source/nix.ncl";
+  #       nix = "/nix/store/...-source/organist.ncl";
   #     };
   #   }
   # Result:
   #   {
   #     organist = {
   #       builders = import "/nix/store/...-source/builders.ncl",
   #       contracts = import "/nix/store/...-source/contracts.ncl",
-  #       nix = import "/nix/store/...-source/nix.ncl",
+  #       nix = import "/nix/store/...-source/organist.ncl",
   #     },
   #   }
   buildLockFile = contents:
@@ -147,9 +147,9 @@
         system = "${system}",
       }
       in
-      let nix = (import "${src}/nickel.lock.ncl").organist in
+      let organist = (import "${src}/nickel.lock.ncl").organist in
 
-      let nickel_expr | nix.contracts.OrganistExpression =
+      let nickel_expr | organist.OrganistExpression =
         import "${src}/${nickelFile}" in
 
       nickel_expr & params
@@ -197,7 +197,7 @@
       nixpkgs = pkgs;
     },
     lockFileContents ? {
-      organist = "${organistSrc}/lib/nix.ncl";
+      organist = "${organistSrc}/lib/organist.ncl";
     },
   }: let
     nickelResult = callNickel {

lib/builders.ncl → lib/nix-interop/builders.ncl

@@ -1,6 +1,6 @@
-let { NickelDerivation, Derivation, NixString, NixEnvironmentVariable, .. } = import "contracts.ncl" in
+let { NickelDerivation, Derivation, NixString, NixEnvironmentVariable, .. } = import "derivation.ncl" in
 
-let lib = import "lib.ncl" in
+let lib = import "builtins.ncl" in
 
 let concat_strings_sep = fun sep values =>
   if std.array.length values == 0 then