Try to fix pagination sdist

doc/cookbook/pagination/Dummy.hs

@@ -1,3 +0,0 @@
-module Main where
-main :: IO ()
-main = return ()

doc/cookbook/pagination/dummy/Pagination.lhs

@@ -0,0 +1,5 @@
+```haskell
+module Main (main) where
+main :: IO ()
+main = return ()
+```

doc/cookbook/pagination/pagination.cabal

@@ -8,20 +8,25 @@ author:              Servant Contributors
 maintainer:          haskell-servant-maintainers@googlegroups.com
 build-type:          Simple
 cabal-version:       >=1.10
+extra-source-files:
+  Pagination.lhs
+  dummy/Pagination.lhs
 tested-with:         GHC==7.8.4, GHC==7.10.3, GHC==8.0.2, GHC==8.2.2, GHC==8.4.2
 
 executable cookbook-pagination
+  main-is:             Pagination.lhs
+  build-tool-depends:  markdown-unlit:markdown-unlit
+  default-language:    Haskell2010
+  ghc-options:         -Wall -pgmL markdown-unlit
+
   if impl(ghc >= 8.0)
-    main-is:             Pagination.lhs
+    hs-source-dirs: .
     build-depends:       base >= 4.8 && <4.12
                        , aeson
                        , servant
                        , servant-server
                        , servant-pagination >= 2.1.0 && < 3.0.0
                        , warp
-    default-language:    Haskell2010
-    ghc-options:         -Wall -pgmL markdown-unlit
-    build-tool-depends: markdown-unlit:markdown-unlit
   else
-    main-is: Dummy.hs
+    hs-source-dirs: dummy
     build-depends: base