diff --git a/lib/filesystem.nix b/lib/filesystem.nix
index f9fd4a981b2d6..5a78bcca4ebd6 100644
--- a/lib/filesystem.nix
+++ b/lib/filesystem.nix
@@ -18,10 +18,7 @@ let
     ;
 
   inherit (lib.filesystem)
-    pathIsDirectory
-    pathIsRegularFile
     pathType
-    packagesFromDirectoryRecursive
     ;
 
   inherit (lib.strings)
@@ -363,30 +360,52 @@ in
       directory,
       ...
     }:
-    assert pathIsDirectory directory;
     let
-      inherit (lib.path) append;
-      defaultPath = append directory "package.nix";
+      # Determine if a directory entry from `readDir` indicates a package or
+      # directory of packages.
+      directoryEntryIsPackage = basename: type:
+        type == "directory" || hasSuffix ".nix" basename;
+
+      # List directory entries that indicate packages in the given `path`.
+      packageDirectoryEntries = path:
+        filterAttrs directoryEntryIsPackage (readDir path);
+
+      # Transform a directory entry (a `basename` and `type` pair) into a
+      # package.
+      directoryEntryToAttrPair = subdirectory: basename: type:
+        let
+          path = subdirectory + "/${basename}";
+        in
+        if type == "regular"
+        then
+        {
+          name = removeSuffix ".nix" basename;
+          value = callPackage path { };
+        }
+        else
+        if type == "directory"
+        then
+        {
+          name = basename;
+          value = packagesFromDirectory path;
+        }
+        else
+        throw
+          ''
+            lib.filesystem.packagesFromDirectoryRecursive: Unsupported file type ${type} at path ${toString subdirectory}
+          '';
+
+      # Transform a directory into a package (if there's a `package.nix`) or
+      # set of packages (otherwise).
+      packagesFromDirectory = path:
+        let
+          defaultPackagePath = path + "/package.nix";
+        in
+        if pathExists defaultPackagePath
+        then callPackage defaultPackagePath { }
+        else mapAttrs'
+          (directoryEntryToAttrPair path)
+          (packageDirectoryEntries path);
     in
-    if pathIsRegularFile defaultPath then
-      # if `${directory}/package.nix` exists, call it directly
-      callPackage defaultPath {}
-    else lib.concatMapAttrs (name: type:
-      # otherwise, for each directory entry
-      let path = append directory name; in
-      if type == "directory" then {
-        # recurse into directories
-        "${name}" = packagesFromDirectoryRecursive {
-          inherit callPackage;
-          directory = path;
-        };
-      } else if type == "regular" && hasSuffix ".nix" name then {
-        # call .nix files
-        "${lib.removeSuffix ".nix" name}" = callPackage path {};
-      } else if type == "regular" then {
-        # ignore non-nix files
-      } else throw ''
-        lib.filesystem.packagesFromDirectoryRecursive: Unsupported file type ${type} at path ${toString path}
-      ''
-    ) (builtins.readDir directory);
+    packagesFromDirectory directory;
 }