Our beloved shapekeys are 'virtual' overrides, they need special
snowflake treatment here as well.
They do not have any override data, from override perspective they are
considered as mere sub-data from their owning ID (mesh, lattice, etc.).
Therefore, we should not copy override data from them, but instead
properly flag those new IDs as `LIB_EMBEDDED_DATA_LIB_OVERRIDE`.
Found while investigating T84373.
Bastien Montagne
69a7015e12