virtual_oss: Move plugins to the sound package

This is the correct place for them, and also issues an incorrect
dependency from utilities to sound.

Fixes: 1b806e607f52 ("packages: Add a sound package")
MFC after: 3 days
Reviewed by: christos, dch
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53015

(cherry picked from commit 419dcdc2c981306b0ded44580977d2e053719b14)