dep_zapdeps: avoid use.mask/force changes (515584)

This patch causes dep_zapdeps to check which USE flags cause a match
to fail, and uses that information to prioritize choices that do not
require changes to use.mask or use.force.

X-Gentoo-Bug: 515584
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=515584