Support cabal-install >= 3.10 config file/index locations

[sternenseemann]

See https://github.com/haskell/cabal/blob/master/release-notes/Cabal-3.10.1.0.md#significant-changes, the config (and index state) can be in XDG style directories instead of ~/.cabal now. The relevant code is implemented upstream in Distribution.Client.Config and is relatively easy to vendor/copy over, e.g. for the state dir.

[Bodigrim]

We use cabal-install-parsers to retrieve Cabal config:

hackage-revdeps/app/Main.hs

Line 55 in 9a8a7a5

cnf <- readConfig

So it depends on haskell-CI/haskell-ci#655 being resolved, but there seem to be compications regarding priority if both paths contain Cabal configs.

For the purposes of hackage-revdeps, we can look for Cabal configs in both locations, look for cfgRepoIndex in all of them and select one with the largest file size. A PR is welcome.

[sternenseemann]

Okay, so per that issue manually setting CABAL_CONFIG can be used to work around the problem if you have everything in the xdg locations?

[Bodigrim]

I think so, but I have not tried.