Broken links for haddock docs

HLS 1.6.1, GHC 8.10.4, Cabal 3.4.0.0, MacOS.

HTML documentation seems to be totally broken for third-party dependencies. Perhaps nobody is using this feature since [it's undocumented](https://github.com/haskell/haskell-language-server/issues/208#issuecomment-919059760).

The `Documentation` link beneath the hover info leads to an almost blank page, with only the `Open on Hackage` link. This URL erroneously refers to the `html` library (which isn't even a dependency of my project) instead of the one we want to browse: e.g. `https://hackage.haskell.org/package/html-1.0.1.2/docs/Data-List-Extra.html#v:splitOn` instead of `https://hackage.haskell.org/package/extra-1.7.10/docs/Data-List-Extra.html#v:splitOn`.