cabal v2-haddock --haddock-for-hackage does not generate hyperlinks to subprojects in the same source tree

[cohomology]

Describe the bug
I am invoking v2-haddock in a haskell repository with multiple subprojects and a cabal.project file. In this project are haskell packages A and B and B depends on A. Then the types in A are not correctly hyperlinked from B.

To Reproduce

git clone https://github.com/cohomology/gi-cairo-render
cd gi-cairo-render
cabal v2-haddock all --builddir=docs --haddock-for-hackage --enable-doc

I get the following warning:

Warning: GI.Cairo.Render.Connector: could not find link destinations for:
    Render

Render is defined in GI.Cairo.Render included in the same git repository.

When I copy the subdirectory gi-cairo-connector out of the repository to somewhere else and invoke the command only for gi-cairo-connector, the link is generated correctly.

I have:

cabal-install version 3.2.0.0
compiled using version 3.2.0.0 of the Cabal library

[Mikolaj]

I wonder if this purely a cabal bug, or if it's in any way helped by the recent progress in the haddock tool: #5949 (comment)

[ulysses4ever]

Compilingwith Cabal 3.8 / GHC 9.4 I only get

Warning: GI.Cairo.Render.Connector: could not find link destinations for:

	- GI.Cairo.Render.Internal.Render

which, indeed, I do not see.

@cohomology could you confirm that this is fixed for you with the latest versions of the tooling? In which case we could close it (along with #6569).