Ignore ports setting NO_PACKAGE when PACKAGE_BUILDING is set
Side effect is that we will no longer mirror their distfiles, sad for them,
but we will no longer spend cpu cycles building them for nothing every week
and have strange errors from dependent ports unable to install NO_PACKAGE
dependencies

Users willing to package those ports can still set FORCE_PACKAGE

Poudriere users can also package by not setting NO_FORCE_PACKAGE in poudriere.conf (by default it's already not set)

Differential Revision: https://reviews.freebsd.org/D670
Reviewed by: bdrewery
With hat: portmgr