Add support for linked collections that are used as input in a Geometry
Nodes modifier. This requires iterating over the geometry nodes modifier
settings, which consists of ID properties. If such an ID property is of
type `IDP_ID`, its pointer is followed and the pointed-to datablock +
its library are visited.
This following of pointers happens in the 'expand' phase, which was only
done for linked library blend files. Since this commit, the old
behaviour of simply looping over all non-`DATA` datablocks of the
to-be-packed blend file is not enough, and datablock expansion is done
for all local datablocks as well.
Not sure when/how that happens, but it happened on a lighting file of the
Spring project, when linking in a nested collection of which parts where
also linked in from other blend files.
This does introduce some not-so-nice things, like having to annotate each
`__init__` function with `-> None`. However, the benefits of having static
type checking in a complex bit of software like BAT outweigh the downsides.
