Add targets/pseudo/bootstrap-packages

Try to compensate for the lack any origin for a DIRDEPS build.

bootstrap-packages will extract the PACKAGE lines from
the tree's Makefiles and use them to populate
targets/packages/Makefile.depend and targets/packages/*/Makefile.depend
so we can at least try to keep the Makefile.depend files throughout
the tree up to date.

This is far from ideal, as we cannot (easily) take into account
options that affect the contents of various packages.

Reviewed by: stevek
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D47065