This moves most of the exporter-related code
from `source/blender/io/alembic/intern`
to `source/blender/io/alembic/exporter`
This is to prepare the Alembic code for the switchover to using
`blender::io::AbstractHierarchyIterator`. When that happens, a few more
files will be added, and having things in a separate 'exporter'
directory makes things less cluttered.
Note that exporting consists of multiple steps (determine export
hierarchy, create Alembic archive, and then write data into it), which
is why the directory is called "exporter", but many of the files are
called "writer".
No functional changes.
Sybren A. Stüvel
89b7f785e6