pkgbase: Let source packages be built in parallel

To build the packages target, we build src and src-sys packages
containing the source code from which the repo was built. These
packages take significantly longer than the others, presumably because
they contain many more files. Because both source packages are built
to satisfy the same target, they end up being built serially. Split
them into separate subtargets so that they can run in parallel. This
saves a couple of minutes on my build machine.

Reviewed by: manu, emaste
MFC after: 1 month
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D46288

(cherry picked from commit d7d5c9efef031aa11a841d1836f7c937dcbe4ec8)