vapoursynth: improve plugin dependency resolution

Some python packages (e.g. functools32) are conditionally disabled for
certain interpreter versions by having them return null instead of a
derivation. `getRecursivePropagatedBuildInputs` fails if such a package
is part of the dependency tree. This commit makes it only recurse into
derivations and ignore everything else.

This also deduplicates the final plugin list to improve startup time.

pkgs/development/libraries/vapoursynth/plugin-interface.nix

@@ -8,10 +8,11 @@ plugins: let
 
   getRecursivePropagatedBuildInputs = pkgs: lib.flatten
     (map
-      (pkg: pkg.propagatedBuildInputs ++ (getRecursivePropagatedBuildInputs pkg.propagatedBuildInputs))
+      (pkg: let cleanPropagatedBuildInputs = lib.filter lib.isDerivation pkg.propagatedBuildInputs;
+        in cleanPropagatedBuildInputs ++ (getRecursivePropagatedBuildInputs cleanPropagatedBuildInputs))
       pkgs);
 
-  deepPlugins = plugins ++ (getRecursivePropagatedBuildInputs plugins);
+  deepPlugins = lib.unique (plugins ++ (getRecursivePropagatedBuildInputs plugins));
 
   pluginsEnv = buildEnv {
     name = "vapoursynth-plugins-env";