packages: move ucl files to a subdirectory

[ivy]

Summary

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.

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.

[ivy]

new version:

• set the comment suffix as a parameter for generate-ucl.lua so it's set properly for both existing packages and packages with a ucl file
• add a description suffix which does the same thing, but with a long-form description of what the subpackage is for
• fix some patterns in generate-ucl.sh which meant certain packages (e.g., libcompat) didn't get their comment suffix set properly

once this is landed, my intention is to mechanically convert Makefile.package
to UCL files and delete it, then there's some additional cleanup we can do
here.