Add -ddump-mod-cycles to -M behaviour
This patch adds a flag -ddump-mod-cycles to the "ghc -M" dependency analyser. The effect of ghc -M -ddump-mod-cycles is to dump a list of cycles foud in the module graph. The display is trimmed so that only dependencies within the cycle are shown; and the list of modules in a cycle is itself sorted into dependency order, so that it is easy to track the chain of dependencies. Open question: should the flag be "-ddump-mod-cycles" or "-optdep-dump-mod-cycles"? For this reason I have not yet added to the documentation.