pkgbase: Don't include pkg by default

We need to put pkg into the pkgbase repository for release builds, but
not for non-release pkgbase builds. Gate the contents of package-pkg
behind INCLUDE_PKG_IN_PKGBASE_REPO=YES and set that when building
releases.

Reviewed by: ivy
MFC after: 6 hours
Sponsored by: https://www.patreon.com/cperciva