Remove dependency of extension manager for migrator.
Keeping load_migrations function for others to use if they desire
but requiring the finder be sent to it in order to use it.
PHPBB3-11386
Allows you to check if the migration is effectively installed
(entirely optionall)
This function is intended to help moving to migrations from a
previous database updater, where some migrations may have been
installed already even though they are not yet listed in the
migrations table.
PHPBB3-9737
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
This is used when a long-running process is needed during an update. For
example, iterating over all posts and applying some transformation. This
allows the process to be broken apart into multiple shorter steps to prevent
hitting the time limit.
PHPBB3-9737
