header cleanups for 6.8

The goal is to get sched.h down to a type only header, so the main thing
happening in this patchset is splitting out various _types.h headers and
dependency fixups, as well as moving some things out of sched.h to
better locations.

This is prep work for the memory allocation profiling patchset which
adds new sched.h interdepencencies.

Testing - it's been in -next, and fixes from pretty much all
architectures have percolated in - nothing major.
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