sched/headers: Reorganize, clean up and optimize kernel/sched/sched.h dependencies

Remove all headers, except the ones required to make this header
build standalone.

Also include stats.h in sched.h explicitly - dependencies already
require this.

Summary of the build speedup gained through the last ~15 scheduler build &
header dependency patches:

Cumulative scheduler (kernel/sched/) build time speedup on a
Linux distribution's config, which enables all scheduler features,
compared to the vanilla kernel:

  _____________________________________________________________________________
 |
 |  Vanilla kernel (v5.13-rc7):
 |_____________________________________________________________________________
 |
 |  Performance counter stats for 'make -j96 kernel/sched/' (3 runs):
 |
 |   126,975,564,374      instructions              #    1.45  insn per cycle           ( +-  0.00% )
 |    87,637,847,671      cycles                    #    3.959 GHz                      ( +-  0.30% )
 |         22,136.96 msec cpu-clock                 #    7.499 CPUs utilized            ( +-  0.29% )
 |
 |            2.9520 +- 0.0169 seconds time elapsed  ( +-  0.57% )
 |_____________________________________________________________________________
 |
 |  Patched kernel:
 |_____________________________________________________________________________
 |
 | Performance counter stats for 'make -j96 kernel/sched/' (3 runs):
 |
 |    50,420,496,914      instructions              #    1.47  insn per cycle           ( +-  0.00% )
 |    34,234,322,038      cycles                    #    3.946 GHz                      ( +-  0.31% )
 |          8,675.81 msec cpu-clock                 #    3.053 CPUs utilized            ( +-  0.45% )
 |
 |            2.8420 +- 0.0181 seconds time elapsed  ( +-  0.64% )
 |_____________________________________________________________________________

Summary:

  - CPU time used to build the scheduler dropped by -60.9%, a reduction
    from 22.1 clock-seconds to 8.7 clock-seconds.

  - Wall-clock time to build the scheduler dropped by -3.9%, a reduction
    from 2.95 seconds to 2.84 seconds.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Peter Zijlstra <peterz@infradead.org>