`cabal haddock-project` does not generate an index file for all the transitive deps

it seems like it generates an index.html thats only the haddocks for your local code, and presumably transitive deps via haddocks.

how would i generate a one true index file? I'd like to keep linkages local for offline hackery, i think i had some scripts for something like this years ago, but this seems like a decent command to have that power

(to be clear, i like how the `./haddock` folder has local docs for all the deps afaict)

Start by reproducing `cabal haddock-project` and inspecting the generated `./haddock` directory, especially its `index.html`. Trace how Haddock pages for local code and transitive dependencies are gathered, then define completion as producing one offline index linking all generated documentation.

Written by the indexing model from the issue text.