MDL-44070 Conditional availability enhancements (2): subsystem/plugin

This commit defines the new /availability root folder, with
/availability/classes, /availability/tests, and
/availability/condition where the condition plugins will live.
Condition plugin prefix is availability_, e.g. availability_date.

Rationale for this organisation:

1. I was originally going to put this in /lib/availability but
   it has been pointed out that putting even more junk in lib
   is probably bad.
2. 'availability' and 'condition' are the two names used in code
   to refer to this system ($CFG->enableavailability).
3. The prefix has to be short enough to allow database tables
   (although in practice I assume that condition plugins will not
   normally contain database tables).