Even though we do not have much of those, this might change in the
future, and in any case having specific functions for this ID type in
generic `BKE_lib` area was really confusing.
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.
