This fixes building machines with the flake-parts module.
Since the inventory merges the machines, the `clan.core.meta` attribute
is now a submodule and conditionally imported.
Also drops the following attributes:
- clan.core.meta.icon
- clan.core.meta.description
In favor of specifying them inside the inventory.
This doesn't use `mkRemovedOptionsModule`, because in our case it would
cause infinite recursion.
e7c6333e06