[1.5.x] Fixed #19401 -- Ensure that swappable model references are case insensitive.

This is necessary because get_model() checks are case insensitive, and if the swapable check isn't, the
swappable logic gets tied up in knots with models that are partially swapped out.

Thanks to chris@cogdon.org for the report and extensive analysis, and Preston for his work on the draft patch.

Backport of c04c03da from trunk.