The sources under kernel/sched/ext/ build as a single translation unit:
build_policy.c includes the source files and headers. An LSP/clangd editor
parses each as a standalone unit, sees no types, and reports a flood of
errors.

Give each header its dependencies and include guard, and have each source
include the headers it uses.

ext.c, arena.c and the ext headers now parse clean standalone. idle.c and
cid.c still reference a few macros and helpers defined in ext.c. The next
patch moves those to shared headers.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

The sched_ext sources had grown to ten ext* files directly under
kernel/sched/. Move them into a new kernel/sched/ext/ subdirectory and drop
the now-redundant ext_ prefix. ext.c/h keep their names.

  kernel/sched/ext.{c,h}       -> kernel/sched/ext/ext.{c,h}
  kernel/sched/ext_internal.h  -> kernel/sched/ext/internal.h
  kernel/sched/ext_types.h     -> kernel/sched/ext/types.h
  kernel/sched/ext_idle.{c,h}  -> kernel/sched/ext/idle.{c,h}
  kernel/sched/ext_cid.{c,h}   -> kernel/sched/ext/cid.{c,h}
  kernel/sched/ext_arena.{c,h} -> kernel/sched/ext/arena.{c,h}

The include paths in build_policy.c and sched.h, the MAINTAINERS glob, and a
few documentation and comment references are updated to match. No code or
symbol changes.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>