Add deprecated compatibility module

compat/Documentation/Haddock.hs

+{-# LANGUAGE PackageImports #-}
+module Documentation.Haddock {-# DEPRECATED "Use @Documentation.Haddock@ from package <http://hackage.haskell.org/package/haddock-api haddock-api> instead!" #-} (
+  module Documentation.Haddock
+) where
+import "haddock-api" Documentation.Haddock

haddock.cabal

@@ -40,6 +40,21 @@ flag in-ghc-tree
   default: False
   manual: True
 
+-- This package re-exports the haddock-api library for compatibility.
+-- The library section will be removed in version 2.16.0.
+library
+  exposed: False
+  default-language: Haskell2010
+  hs-source-dirs: compat
+  ghc-options: -Wall
+  build-depends:
+      -- NOTE: Don't bump base dependency!  Remove library section instead!
+      base >= 4.3 && < 4.8
+    , haddock-api
+  exposed-modules: Documentation.Haddock
+  if flag(in-ghc-tree)
+    buildable: False
+
 executable haddock
   default-language:     Haskell2010
   main-is:              Main.hs