factor out nix code into new files via makeScope

This makes the Nix code a lot easier to reason about.

nix/pkgs/book/default.nix

@@ -0,0 +1,34 @@
+# Keep sorted
+{ default
+, inputs
+, mdbook
+, stdenv
+}:
+
+stdenv.mkDerivation {
+  pname = "${default.pname}-book";
+  version = default.version;
+
+
+  src = let filter = inputs.nix-filter.lib; in filter {
+    root = inputs.self;
+
+    # Keep sorted
+    include = [
+      "book.toml"
+      "conduit-example.toml"
+      "debian/README.md"
+      "docs"
+      "README.md"
+    ];
+  };
+
+  nativeBuildInputs = [
+    mdbook
+  ];
+
+  buildPhase = ''
+    mdbook build
+    mv public $out
+  '';
+}