Now that the IWYU cleanup has been merged compile all, not just a couple, of
the builtin modules independent of builtin.cpp. That is, no longer `#include
builtin_NAME.cpp` in builtin.cpp. This is more consistent, more in line with
what developers expect, and is likely to reduce mistakes.
Reduces lint errors from 384 to 336 (-13%). Line count from 6307 to 4988 (-21%).
Another step in resolving issue #2902.
