_slot_confict_backtrack: minimize conflict atoms (bug 743631)

Prefer choices that minimize conflict atoms, so that choices
which satisfy all parents are preferred. This reduces the
minimum necessary backtrack tries from 21 to 7 for the unit
test related to bug 743115.

BUG=b:237827425, b:240088960
TEST=CQ

Bug: https://bugs.gentoo.org/743115
Bug: https://bugs.gentoo.org/743631
Signed-off-by: Zac Medico <zmedico@gentoo.org>
(cherry picked from commit f64310749f176f8921b72ce282b4294efe81c3f0)
Change-Id: Ie8ad6d2d85a04eee3ffefc6e4ceb4bbc2b9d9470
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/portage_tool/+/3792750
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Alex Klein <saklein@chromium.org>
Tested-by: Alex Klein <saklein@chromium.org>
2 files changed