They only contain private functions and don't need to be installed or
included by gimp_pdb_headers.h.
The PDB generation part is done by adding a "lib_private" variable
that can be set on PDB groups which should not be public API; the rest
is manual Makefile fiddling.
It never belonged inside "tools". Also rename its "pdb" subdirectory
to "groups". This had to happen before 2.10 so cherry-picking between
branches doesn't become a nightmare in the future.