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Alembic import: load face-varying normals

Loop normals are called 'Face-varying normals' in Alembic. Before this
commit, the existence of such normals was used to enable smooth shading.
This is incorrect, as the normals could encode flat faces just as well.

This commit adds the loading of these normals as custom loop normals. It
then also enables Auto-Smoothing on the mesh (which is a bit of a
misnomer and indicates to Blender that the custom normals should be
used).

Fixes the glitching described in T65959.

Differential Revision: https://developer.blender.org/D5191