Fix hyperlinks to external items and modules (#1482)

Fixes #1481.

There were two bugs in this:
* We were assuming that we were always getting a relative path to the
  module in question, while Nix gives us file:// URLs sometimes. This
  change checks for those and stops prepending `..` to them.
* We were not linking to the file under the module. This seems
  to have been a regression introduced by #977. That is, the URLs were
  going to something like
  file:///nix/store/3bwbsy0llxxn1pixx3ll02alln56ivxy-ghc-9.0.2-doc/share/doc/ghc/html/libraries/base-4.15.1.0/src
  which does not have the appropriate HTML file or fragment for the item
  in question at the end.

There is a remaining instance of the latter bug, but not in the
hyperlinker: the source links to items reexported from other modules are
also not including the correct file name. e.g. the reexport of Entity in
esqueleto, from persistent.

NOTE: This needs to get tested with relative-path located modules. It seems
correct for Nix based on my testing.

Testing strategy:

```
nix-shell '<nixpkgs>' --pure -A haskell.packages.ghc922.aeson
mkdir /tmp/aesonbuild && cd /tmp/aesonbuild
export out=/tmp/aesonbuild/out
genericBuild

ln -sf $HOME/co/haddock/haddock-api/resources .
./Setup haddock --with-haddock=$HOME/path/to/haddock/exec --hyperlink-source
```

Author: lf-

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

@@ -23,6 +23,7 @@ import System.FilePath.Posix ((</>))
 
 import qualified Data.Map as Map
 import qualified Data.Set as Set
+import qualified Data.List as List
 
 import Text.XHtml (Html, HtmlAttr, (!))
 import qualified Text.XHtml as Html
@@ -248,14 +249,20 @@ hyperlink (srcs, srcs') ident = case ident of
     Left name -> externalModHyperlink name
 
   where
+    -- In a Nix environment, we have file:// URLs with absolute paths
+    makeHyperlinkUrl url | List.isPrefixOf "file://" url = url
+    makeHyperlinkUrl url = ".." </> url
+
     internalHyperlink name content =
         Html.anchor content ! [ Html.href $ "#" ++ internalAnchorIdent name ]
 
     externalNameHyperlink name content = case Map.lookup mdl srcs of
         Just SrcLocal -> Html.anchor content !
             [ Html.href $ hypSrcModuleNameUrl mdl name ]
-        Just (SrcExternal path) -> Html.anchor content !
-            [ Html.href $ spliceURL Nothing (Just mdl) (Just name) Nothing (".." </> path) ]
+        Just (SrcExternal path) ->
+          let hyperlinkUrl = makeHyperlinkUrl path </> hypSrcModuleNameUrl mdl name
+           in Html.anchor content !
+                [ Html.href $ spliceURL Nothing (Just mdl) (Just name) Nothing hyperlinkUrl ]
         Nothing -> content
       where
         mdl = nameModule name
@@ -264,8 +271,10 @@ hyperlink (srcs, srcs') ident = case ident of
         case Map.lookup moduleName srcs' of
           Just SrcLocal -> Html.anchor content !
             [ Html.href $ hypSrcModuleUrl' moduleName ]
-          Just (SrcExternal path) -> Html.anchor content !
-            [ Html.href $ spliceURL' Nothing (Just moduleName) Nothing Nothing (".." </> path) ]
+          Just (SrcExternal path) ->
+            let hyperlinkUrl = makeHyperlinkUrl path </> hypSrcModuleUrl' moduleName
+             in Html.anchor content !
+                  [ Html.href $ spliceURL' Nothing (Just moduleName) Nothing Nothing hyperlinkUrl ]
           Nothing -> content