Make haskellModuleRulesToRuleInfos more strict when calling haskellModulesToModuleData

[googleson78]

In haskellModuleRulesToRuleInfos, when we convert a module rule to rule data, we expect to get at most 1 mod data (and the code doesn't handle the case in which it returns more than one). Since this is post-condition that's present there, we should assert (by fatalling) if this goes wrong, because it happen to be very wrong.

It might also be worth making a specialised haskellModuleToModuleData which doesn't return a slice too, where we can also place the assert.