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.

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