Can't access __HADDOCK_VERSION__

Following the discussion in #9177

@FinleyMcIlwaine I want to have access to `__HADDOCK_VERSION__`; alas, using the mechanism described in #9177 doesn't seem to work.

To be specific, I created a `project.cabal` file, with contents:

```
packages: .

package *
documentation: true
haddock-version-cpp: True
```

but this doesn't seem to make `__HADDOCK_VERSION__` defined as a macro in source files. Any advice?

Start with the `project.cabal` configuration shown in the issue and read the discussion in #9177 to understand the intended `haddock-version-cpp` mechanism. Trace how that setting is handled for source files; done means `__HADDOCK_VERSION__` is defined as a macro when the configuration is used.

Written by the indexing model from the issue text.