nixpkgs-manual: avoid lib.fileset

I don’t love this, and I’m not convinced it doesn’t have the
same pitfalls as filesets, but see the `flattenReferencesGraph`
commit for reasoning.

It may be better to ban the relevant builtins entirely and just move
things into subdirectories when needed, but I didn’t want to do
more surgery to this part of the tree than necessary to solve the
immediate problem.

doc/doc-support/lib-function-docs.nix

@@ -90,10 +90,7 @@
 stdenvNoCC.mkDerivation {
   name = "nixpkgs-lib-docs";
 
-  src = lib.fileset.toSource {
-    root = ../..;
-    fileset = ../../lib;
-  };
+  src = ../../lib;
 
   nativeBuildInputs = [
     nixdoc
@@ -101,6 +98,8 @@ stdenvNoCC.mkDerivation {
   ];
 
   installPhase = ''
+    cd ..
+
     export NIX_STATE_DIR=$(mktemp -d)
     nix-instantiate --eval --strict --json ${./lib-function-locations.nix} \
       --arg nixpkgsPath "./." \

doc/doc-support/package.nix

@@ -25,16 +25,22 @@ stdenvNoCC.mkDerivation (
 
     nativeBuildInputs = [ nixos-render-docs ];
 
-    src = lib.fileset.toSource {
-      root = ../.;
-      fileset = lib.fileset.unions [
-        (lib.fileset.fileFilter (file: file.hasExt "md" || file.hasExt "md.in") ../.)
-        ../style.css
-        ../anchor-use.js
-        ../anchor.min.js
-        ../manpage-urls.json
-        ../redirects.json
-      ];
+    src = lib.cleanSourceWith {
+      src = ../.;
+      filter =
+        path: type:
+        type == "directory"
+        || lib.hasSuffix ".md" path
+        || lib.hasSuffix ".md.in" path
+        || lib.elem path (
+          map toString [
+            ../style.css
+            ../anchor-use.js
+            ../anchor.min.js
+            ../manpage-urls.json
+            ../redirects.json
+          ]
+        );
     };
 
     postPatch = ''