Move the DeferredUpdates hierarchy to its own directory

The grouping makes at least as much sense as job/, and certainly makes
more sense than cache/. With directories named after base classes, it is
fairly easy to tell what should go where. The grouping of
DeferredUpdates, DataUpdate and CallableUpdate would surely be
uncontroversial.

The move of SearchUpdate out of search/ demonstrates the conflict between
arrangement by module versus arrangement by type, which is the most
difficult design question here. I think arrangement by type is more
consistent with e.g. the arrangement of the core root, i.e. tests/,
resources/, maintenance/, etc. where a given feature will have its files
split up into a mostly type-based hierarchy.

I also tidied up AutoLoader.php by moving includes/content to the correct
location, sorted alphabetically by subdirectory.

Verified with AutoLoaderTest.

Change-Id: Ib369411d0caca38e72978084aa57348f1b892ed0