pkg: move active_list structure out of pkg_t

pkg: move active_list structure out of pkg_t

The active list head is only used by a tiny fraction of the allocated
package structures in memory so do not waste heap memory by allocating
space for it in all loaded packages.

Instead allocate active list heads dynamically where needed and point
them to the corresponding packages.

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