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
```

(cherry picked from commit ab53ccf0)