gnu: pandoc-citeproc: Separate outputs.

This removes unnecessary references to Haskell packages for the statically
built executable.

* gnu/packages/haskell-xyz.scm (pandoc-citeproc)[outputs]: Add "lib" and "doc"
outputs.

gnu/packages/haskell-xyz.scm

@@ -9581,6 +9581,7 @@ (define-public pandoc-citeproc
          ("pandoc" ,pandoc "lib")
          ,@all-static-inputs
          ,@direct-inputs)))
+    (outputs '("out" "lib" "static" "doc"))
     (synopsis "Pandoc filter for bibliographic references")))
 
 (define-public ghc-pandoc-types