add a way to exclude a folder from `default`/`all` without disabling that folder completely

This is kind of related to #9690.

We have a sub folder of our monorepo in which we put experimental code. This code might not compile. Or it could compile, but requires to have a very specific environment to do so. Currently we can make sure that folder won't be built during a dune build at the root by using (dirs :standard \ experimental). But by doing that we make it impossible for someone to work on code that lives experimental.

It would be nice to have a softer exclusion than what dirs does. To have a folder which wouldn't be picked by dune build, which wouldn't expose its executables/libraries/ppx to the rest of the repo (while still being able to consume the exe/libs/ppx of the rest of the repo), but could still be built if necessary.

Start by reading related issue #9690 and tracing how (dirs :standard \\ experimental) affects root dune build and folder visibility. Define the behavior needed for default/all: experimental code is excluded and does not expose artifacts, while remaining able to consume the rest of the repository and be built explicitly. Done means this behavior is specified and covered for the described monorepo case.