Merge t_pbc definition into pbc.h

- Remove types/pbc.h, and merge its contents into pbc.h.
- Make t_pbc forward-declarable, and use this in header that only
  included one of the pbc.h files to just get this definition.
- Add some missing pbc.h includes.
- Fix some other include dependencies that were previously covered
  transitively.
- Minor cleanup in touched files.

Change-Id: I0a69980b244c23905c44448c1a095d497807cd04