libopkg: implement lightweight package listing logic

libopkg: implement lightweight package listing logic

Instead of building a complete package dependency tree internally, use a
lightweight list data structure to gather and sort package names, version and
descriptions.

This reduced the "opkg list" memory footprint on x86/64 from ~8MB to ~1.5MB.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>