Refactor do-package aka subpackages step 2

Now each files generated are explicit targets depending on the files
This would allow to simply loop over a list of packages for subpackages to
create them.

Remove the code for transition from pkg_install to pkg packages

The latest symlink is now a specific target only defined when needed

Reviewed by: swills
Approved by: portmgr (swills)
Differential Revision: https://reviews.freebsd.org/D10323