The PBVH struct is now called `bke::pbvh::Tree`. Expanding the acronym
in the file name just a little should help developers find things and make
the connection to the "paint" concept that loosely ties sculpt mode and
other painting modes together a little stronger.
This rename also lets us replace the weird `_api.hh` historical part of
the file name without reusing the old `BKE_pbvh.hh` file name, which
would have probably made understanding the git history a bit harder.
Pull Request: https://projects.blender.org/blender/blender/pulls/129684
0be2535369