`--hoogle` flag of `haddock-project` makes no difference

**Describe the bug**
I am not exactly sure what the expected outcome of `cabal haddock-project --hoogle` is, but from what I can tell, the `--hoogle` flag makes no difference at all.

**To Reproduce**
Steps to reproduce the behavior:
```bash
$ cabal haddock-project --output=docs
$ cabal haddock-project --output=docs-hoogle --hoogle
$ meld docs docs-hoogle # or any other diff tool that can diff directories
```

**Expected behavior**
`--hoogle` flag should have an effect

**System information**
- Operating system: NixOS
- `cabal`: 3.12.1.0
- `ghc`: 9.6.5

Reproduce the issue with the two `cabal haddock-project` commands and compare the `docs` and `docs-hoogle` directories. Trace the `--hoogle` handling from the `haddock-project` command entry point, then add coverage showing that the flag changes the generated output; the expected behavior still needs clarification.

Written by the indexing model from the issue text.