This is required so that when migrations are reverted we can check through
all installed migrations and make sure that all dependencies are handled
properly and so that we are only required to load the migrations files
that could be dependent on the ones installed.
I believe in normal proper use the old way might have worked, but in case
something happens and an unrelated migration file is installed, but cannot
be loaded, this makes sure we do not stop everything unless we absolutely
must (one of those files is dependent on something we want to revert).
PHPBB3-9737
