Setting (and loading) preferences for "sub-packages" with shared Manifest

[NHDaly]

At RelationalAI, we have organized our large mono-repo codebase using "sub-packages," a newly supported pattern in Pkg (since 1.9) where the codebase is split into several packages, and the top-level project dev's those subpackages.
In each subpackage, we set the manifest = field in that subpackage's Project.toml to point back to the parent Manifest, like manifest = "../../Manifest.toml".

The structure looks something like this:

Project.toml
Manifest.toml
src/
    - ...
test/
    - ...
packages/
    - PkgA/
        - Project.toml    # Links to the top-level manifest via `manifest = "../../Manifest.toml"`
        - src/
        - test/

This has worked quite well for us! If you have the top-level project activated, you can run Pkg.test("PkgA") to test the subpackage. If you are developing on that package, you can have either the top-level or the sub-package activated and using PkgA works as expected. If you want to change dependencies, you can ]activate packages/PkgA, then ]add dep or whatever, and the dependencies are instantiated in the top-level shared Manifest.

But we're starting to use Preferences.jl a bit, and we're seeing some issues. If you create a LocalPreferences.toml file in the top-level package, then the preferences are picked up by using PkgA. But if you run Pkg.test("PkgA"), or ReTestItems.runtests("packages/PkgA"), the preferences are not picked up. If you put the LocalPreferences file only in the package itself, it works for testing, but they aren't picked up for normal development.

---

I would like to propose teaching Preferences.jl (or is it Base.get_preferences()?) to automatically look for the preferences file next to the activate Manifest.toml, not just next to the current Project.toml.

This would make Preferences work well with this "sub-packages" pattern, much like the rest of Pkg has been made to work.

Does that seem acceptable to you all?
Thank you!