This replaces header include guards with `#pragma once`.
A couple of include guards are not removed yet (e.g. `__RNA_TYPES_H__`),
because they are used in other places.
This patch has been generated by P1561 followed by `make format`.
Differential Revision: https://developer.blender.org/D8466
Having functions defined in `BKE_lib_id.h` implemented into
`lib_remap.c` was confusing at best.
Besides trivial code splitting and header includes cleanup, had to add a
new `lib_intern.h` header for callbacks used by both remapping and
deletion code.
