zlib: Move to a new zlib package

zlib is a standalone third-party component, and deserves its own
package rather than living in runtime. For example, this will make
future security updates less invasive. This also means there's no
dependency on runtime for ports that just require zlib, which is
useful for service jails.

MFC after: 3 days
Reviewed by: bapt, emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53058