pkgbase: move libedit into the -runtime package

libedit isn't as foundational as the rest of -clibs, but various bits
of -runtime do need it. Give it a new home over in -runtime.

This also fixes the libedit build to group the readline bits into the
same package as the libedit that implements them.

Reviewed by: ivy
MFC after: 2 days (pkgbase movement)
Differential Revision: https://reviews.freebsd.org/D52787