packages: Convert world to a subdir build

Instead of driving the world package build from Makefile.inc1,
use a subdir build where each package has a subdirectory under
packages/ using the new <bsd.p

packages: Convert world to a subdir build

Instead of driving the world package build from Makefile.inc1,
use a subdir build where each package has a subdirectory under
packages/ using the new <bsd.pkg.mk>.

Convert some metadata that was previously in the UCL files (e.g.
sets and dependencies) to Makefile variables.

Build the packages under objdir (not repodir), and use the new
stagepackages target to copy them to repodir when creating the
repository.

Determine an explicit list of packages to build in packages/Makefile
based on enabled src.conf options, and add logic to abort the build
if we attempt to build an empty package. This inverts the previous
logic in Makefile.inc1 which would simply skip empty packages.

There are a few advantages to doing it this way:

* The package build works more like the rest of the build system,
so it's more accessible to developers.

* We can customise the packages we build based on src.conf options,
e.g. skipping a package entirely, or adjusting its dependencies
based on what it actually requires.

* We have a specific list of packages that we want to build, and an
unexpectedly missing package results in a build error, instead of
silently producing a broken repository.

* It's possible to build (and in the future, install) an individual
package without having to rebuild the entire repository.

This doesn't apply to the dtb, kernel-* or src-* packages; those
have their own build systems in Makefile.inc1 and will be converted
later.

MFC after: 4 weeks (stable/15 only)
Reviewed by: jlduran, sjg, brooks
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D56087

show more ...