haddock: Fix hyperlinker source urls (#24907)

This fixes a bug introduced by f56838c36235febb224107fa62334ebfe9941aba Links to
external modules in the hyperlinker are uniformly generated using splicing the
template given to us instead of attempting to construct the url in an ad-hoc manner.

haddock-api/src/Haddock/Backends/Hyperlinker/Renderer.hs

@@ -275,7 +275,7 @@ hyperlink (srcs, srcs') ident = case ident of
         Html.anchor content
           ! [Html.href $ hypSrcModuleNameUrl mdl name]
       Just (SrcExternal path) ->
-        let hyperlinkUrl = makeHyperlinkUrl path </> hypSrcModuleNameUrl mdl name
+        let hyperlinkUrl = hypSrcModuleUrlToNameFormat $ makeHyperlinkUrl path
          in Html.anchor content
               ! [Html.href $ spliceURL (Just mdl) (Just name) Nothing hyperlinkUrl]
       Nothing -> content
@@ -288,7 +288,7 @@ hyperlink (srcs, srcs') ident = case ident of
           Html.anchor content
             ! [Html.href $ hypSrcModuleUrl' moduleName]
         Just (SrcExternal path) ->
-          let hyperlinkUrl = makeHyperlinkUrl path </> hypSrcModuleUrl' moduleName
+          let hyperlinkUrl = makeHyperlinkUrl path
            in Html.anchor content
                 ! [Html.href $ spliceURL' (Just moduleName) Nothing Nothing hyperlinkUrl]
         Nothing -> content

haddock-api/src/Haddock/Backends/Hyperlinker/Utils.hs

@@ -13,6 +13,7 @@ module Haddock.Backends.Hyperlinker.Utils
   , hypSrcModuleUrlFormat
   , hypSrcModuleNameUrlFormat
   , hypSrcModuleLineUrlFormat
+  , hypSrcModuleUrlToNameFormat
   , spliceURL
   , spliceURL'
 
@@ -82,6 +83,9 @@ hypSrcModuleNameUrlFormat = hypSrcModuleUrlFormat ++ "#" ++ nameFormat
 hypSrcModuleLineUrlFormat :: String
 hypSrcModuleLineUrlFormat = hypSrcModuleUrlFormat ++ "#" ++ lineFormat
 
+hypSrcModuleUrlToNameFormat :: String -> String
+hypSrcModuleUrlToNameFormat url = url ++ "#" ++ nameFormat
+
 moduleFormat :: String
 moduleFormat = "%{MODULE}.html"