for packages with specific dependencies, add the dependencies in their
UCL files instead of listing them in generate-ucl.sh.

generate-ucl.sh has logic to add automatic subpackage dependencies, so
e.g. "FreeBSD-foo-dev" depends on "FreeBSD-foo". move this into
generate-ucl.lua instead.

this means we no longer need to pre-process the UCL in generate-ucl.sh,
and we can undo the hack where it calls generate-ucl.lua with the same
filename as both input and output file.