doc: extract epub manual stub into its own package

doc/default.nix

@@ -10,47 +10,7 @@ let
     inherit nixpkgs;
   };
 
-  epub = pkgs.runCommand "manual.epub" {
+  epub = callPackage ./doc-support/epub.nix { };
-    nativeBuildInputs = with pkgs; [ libxslt zip ];
-
-    epub = ''
-      <book xmlns="http://docbook.org/ns/docbook"
-            xmlns:xlink="http://www.w3.org/1999/xlink"
-            version="5.0"
-            xml:id="nixpkgs-manual">
-        <info>
-          <title>Nixpkgs Manual</title>
-          <subtitle>Version ${pkgs.lib.version}</subtitle>
-        </info>
-        <chapter>
-          <title>Temporarily unavailable</title>
-          <para>
-            The Nixpkgs manual is currently not available in EPUB format,
-            please use the <link xlink:href="https://nixos.org/nixpkgs/manual">HTML manual</link>
-            instead.
-          </para>
-          <para>
-            If you've used the EPUB manual in the past and it has been useful to you, please
-            <link xlink:href="https://github.com/NixOS/nixpkgs/issues/237234">let us know</link>.
-          </para>
-        </chapter>
-      </book>
-    '';
-
-    passAsFile = [ "epub" ];
-  } ''
-    mkdir scratch
-    xsltproc \
-      --param chapter.autolabel 0 \
-      --nonet \
-      --output scratch/ \
-      ${pkgs.docbook_xsl_ns}/xml/xsl/docbook/epub/docbook.xsl \
-      $epubPath
-
-    echo "application/epub+zip" > mimetype
-    zip -0Xq -b "$TMPDIR" "$out" mimetype
-    cd scratch && zip -Xr9D -b "$TMPDIR" "$out" *
-  '';
 
   # NB: This file describes the Nixpkgs manual, which happens to use module
   #     docs infra originally developed for NixOS.

doc/doc-support/epub.nix

@@ -0,0 +1,53 @@
+{
+  lib,
+  runCommand,
+  docbook_xsl_ns,
+  libxslt,
+  zip,
+}:
+runCommand "manual.epub"
+  {
+    nativeBuildInputs = [
+      libxslt
+      zip
+    ];
+
+    epub = ''
+      <book xmlns="http://docbook.org/ns/docbook"
+            xmlns:xlink="http://www.w3.org/1999/xlink"
+            version="5.0"
+            xml:id="nixpkgs-manual">
+        <info>
+          <title>Nixpkgs Manual</title>
+          <subtitle>Version ${lib.version}</subtitle>
+        </info>
+        <chapter>
+          <title>Temporarily unavailable</title>
+          <para>
+            The Nixpkgs manual is currently not available in EPUB format,
+            please use the <link xlink:href="https://nixos.org/nixpkgs/manual">HTML manual</link>
+            instead.
+          </para>
+          <para>
+            If you've used the EPUB manual in the past and it has been useful to you, please
+            <link xlink:href="https://github.com/NixOS/nixpkgs/issues/237234">let us know</link>.
+          </para>
+        </chapter>
+      </book>
+    '';
+
+    passAsFile = [ "epub" ];
+  }
+  ''
+    mkdir scratch
+    xsltproc \
+      --param chapter.autolabel 0 \
+      --nonet \
+      --output scratch/ \
+      ${docbook_xsl_ns}/xml/xsl/docbook/epub/docbook.xsl \
+      $epubPath
+
+    echo "application/epub+zip" > mimetype
+    zip -0Xq -b "$TMPDIR" "$out" mimetype
+    cd scratch && zip -Xr9D -b "$TMPDIR" "$out" *
+  ''