Fixes the packing operators that use ED_uvedit_pack_islands_multi
Also fixes UV Select Similar with hidden UV islands.
Packing operators using GEO_uv_parametrizer should remain unchanged.
Add a check to BM_elem_flag_test(efa, BM_ELEM_HIDDEN).
Note that BM_mesh_calc_face_groups doesn't easily support XOR of flags,
requiring logic to be moved to a preprocess step on BM_ELEM_TAG.
Regression in rBe3075f3cf7ce.
Differential Revision: https://developer.blender.org/D17055
6672b5373f