Namespaced packages in a repo with a shared `.isort.cfg` are misidentified as third-party

Consider a git repo that contains two namespaced packages, my.first.library and my.second.library, with a single shared .isort.cfg:

root
  .git/
  .isort.cfg
  my_first_library/
    my/
      first/
        library/
          __init__.py
  my_second_library/
    my/
      second/
        library/
          __init__.py

The fact of containing two libraries is actually incidental; AFAICT the issue is the extra directory in there. But probably the most common use case for this would be a monorepo containing multiple libraries.

In this case, running isort . inside my_first_library/ will erroneously identify imports from my.first.library as third-party. The same is true in my_second_library/ with imports from my.second.library. Copying .isort.cfg into my_first_library/ is sufficient to solve this, but in our case we have a monorepo with dozens and dozens of python libraries and would rather not duplicate our .isort.cfg if we don't have to. A symlink also works, but we're hoping for a built-in solution.

I know this is a serious edge case, but is there any way to give isort a hint about where the root directory of the package is to help it identify first-party packages?

Note that this works fine for non-namespaced packages, so it has the feel of a bug, but it's also probably low priority.

Reproduce the reported layout by running isort . from each library directory with the shared .isort.cfg; then trace the package and configuration-root detection used for first-party classification. Done means namespaced imports are recognized as first-party without duplicating or symlinking the configuration, while non-namespaced behavior remains unchanged.