dep_zapdeps: handle circular deps with --onlydeps

This fixes a case with --onlydeps were dep_zapdeps would pull in an
avoidable direct circular dependency on an onlydeps node. The logic
changes only apply to --onlydeps, so there's no chance of regressions
for cases when --onlydeps is not enabled.

X-Gentoo-Bug: 524916
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=524916
Acked-by: Alexander Berntsen <bernalex@gentoo.org>