make submodule an alternate

[basbebe]

Is your feature request related to a problem? Please describe.

There are applications like stackline that install as a submodule.
They might also be very system-specific (macOS in this case) and it would be great to have the possibility to make them alternates as well.

Describe the solution you'd like

Maybe the folder of the submodules could get the attribute as a suffix?

Additional context

This is not a duplicate of #78 I think because that one specifically is about alternates within submodules.

[github-actions]

This issue has been labeled as stale because it has been open 60 days with no activity. Remove stale label or comment or this will be closed in 7 days.

[github-actions]

This issue has been labeled as stale because it has been open 60 days with no activity. Remove stale label or comment or this will be closed in 7 days.