packages: move ucl files to a subdirectory

We want to have a UCL file for every package, to set fields like
comment, desc, license, etc.; to avoid cluttering release/packages with
~200 new files, move these to the ucl/ directory.

Add a better description for the yp package to demonstrate how this will
work in practice.

Clean up the handling of the package comment suffix for generated
packages (dev, lib32, etc.) and do the UCL modifications in
generate-ucl.lua, so we don't overwrite the ucl's comment.

Add a long-form suffix to the description for generated packages to
explain what they do.

Eventually, we should require the ucl file exists (try=false) to prevent
people accidentally creating new packages with a typo, but for now, keep
the ucl optional.

Reviewed by: kevans, des, bapt, emaste
Approved by: kevans (mentor), des (mentor)
Differential Revision: https://reviews.freebsd.org/D50160