Verify compliance w/ PMS rules about all USE conditionals relying on variables defined in IUSE effective.

See https://github.com/pkgcore/pkgcheck/commit/f2d4e371da2bfa834644973d4dd0bede50deafb4 and https://bugs.gentoo.org/921841 .  pkgcheck now checks for this, but pkgcore should be flagging it as it's building the depset.