A number of small changes:

- Some comment typos fixed.
- The main function for cycle detection is now called
  `cycleDetectionPhase`, in analogy with all the other
  phases.

I've run a superficial performance test trying to install
all of Hackage on a clean db with ghc-7.10.3. This is not
likely to trigger any situations where cycle detection
actually kicks in, but it confirms in general that there is
no negative performance (or correctness) impact for the
common case.

I've also considered moving the cycle detection phase to
"earlier" in the solver, but after performance testing, decided
against it, and documented the decision and the reasons in
the code.