Solver: Check for cycles after every step.

[grayjay]

Previously, the solver only checked for cycles after it had already found a
solution. That reduced the number of times that it performed the check in the
common case where there were no cycles. However, when there was a cycle, the
solver could spend a lot of time searching subtrees that already had a cyclic
dependency and therefore could not lead to a solution. This is part of
#3824.

Changes in this commit:

• Store the reverse dependency map on all choice nodes in the search tree, so
  that 'detectCyclesPhase' can access it at every step.
• Check for cycles incrementally at every step. Any new cycle must contain the
  current package, so we just check whether the current package is reachable
  from its neighbors.
• If there is a cycle, we convert the map to a graph and find a strongly
  connected component, as before.
• Instead of using the whole strongly connected component as the conflict set,
  we select one cycle. Smaller conflict sets are better for backjumping.
• The incremental cycle detection automatically fixes a bug where the solver
  filtered out the message about cyclic dependencies when it summarized the full
  log. The bug occurred when the failure message was not immediately after the
  line where the solver chose one of the packages involved in the conflict. See
  new-build dependency solver failure with GHC 7.0 #4154.

I tried several approaches and compared performance when solving for
packages with different numbers of dependencies. Here are the results. None of
these runs involved any cycles, so they should have only tested the overhead of
cycle checking. I turned off assertions when building cabal.

Index state: index-state(hackage.haskell.org) = 2016-12-03T17:22:05Z
GHC 8.0.1

Runtime in seconds:

Packages	Search tree depth	Trials	master	This PR	#1	#2
yesod	343	3	2.00	2.00	2.13	2.02
yesod gi-glib leksah	744	3	3.21	3.31	4.10	3.48
phooey	66	3	3.48	3.54	3.56	3.57
Stackage nightly snapshot	6791	1	186	193	357	191

Total memory usage in MB, with '+RTS -s':

Packages	Trials	master	This PR	#1	#2
yesod	1	189	188	188	198
yesod gi-glib leksah	1	257	257	263	306
Stackage nightly snapshot	1	1288	1338	1432	12699

#1 - Same as master, but with cycle checking (Data.Graph.stronglyConnComp) after
every step.
#2 - Store dependencies in Distribution.Compat.Graph in the search tree, and
check for cycles containing the current package at every step.

/cc @kosmikus

[mention-bot]

@grayjay, thanks for your PR! By analyzing the history of the files in this pull request, we identified @kosmikus, @edsko and @dcoutts to be potential reviewers.

[ezyang]

LGTM, thank you!

[grayjay]

Thanks!