Factoring out usages of Deps as list of installed libraries

Prefer either of the following for stuff like this:

```haskell
satisfyConstraint (Range lower upper) version =
    satisfyLower lower version && satisfyUpper upper version

satisfyConstraint constraint version =
    let (Range lower upper) = constraint in
    satisfyLower lower version && satisfyUpper upper version
```

I think the first one is the best way. Is the idea that this will be extended later, so this is future proof?