parallel_emerge: Crack all counter-plan dependencies.

parallel_emerge: Crack all counter-plan dependencies.

Previously, parallel_emerge only broke counter-plan dependencies if they were
mutually cyclic. This doesn't work, because these deps might be involved in
an unrelated cycle.

To fix this, we break all counter-plan dependencies that are involved in any
cycle. This fixes make_chroot --fast, which currently fails with a deadlock.

Besides the above, I've also added additional cycle-checking to the install
plan generation code so that cyclic dependencies are caught earlier. I also
cleaned up FindCyclesAtNode to be a bit more understandable -- I wasn't 100% sure
that it was actually correct so I cleaned it up so that it was easier for me to
understand that it is in fact correct. Not sure that this part fixes any bugs,
but it's easier for me to analyze the behavior of FindCyclesAtNode now.

TEST=make_chroot --fast (Note that make_chroot --fast is deprecated, but it's
     good to fix the cycle cracking anyway.)
BUG=chromium-os:5795

[Nick Sanders, 2010-08-17 04:18:38]

lgtm

might be good to add a comment about killing all counterplan deps, rather than
just one.