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

Makefile.inc1

@@ -2047,6 +2047,7 @@ update-packages: .PHONY
 	${_+_}${MAKE} -C ${.CURDIR} ${PKGMAKEARGS} real-update-packages
 
 package-pkg: _pkgbootstrap _repodir .PHONY
+.if defined(INCLUDE_PKG_IN_PKGBASE_REPO)
 .if exists(${PORTSDIR})
 	rm -rf /tmp/ports.${TARGET} || :
 	env ${WMAKEENV} SRCDIR=${.CURDIR} PORTSDIR=${PORTSDIR} REVISION=${_REVISION} \
@@ -2055,6 +2056,7 @@ package-pkg: _pkgbootstrap _repodir .PHONY
 		OSVERSION="${SRCRELDATE}" \
 		sh ${.CURDIR}/release/scripts/make-pkg-package.sh
 .endif
+.endif
 
 real-packages:	stage-packages create-packages sign-packages .PHONY