Strict import style: Build Modules

Not much to do here since a lot of this has been done more recently.

`Build.File` and `Build.Flags` have long unqualifed import lists that I think are worth it and in `Build.Source` I did a couple types unqualified while making all of the normal identifiers qualified.
I sort of think this should be imported as `RWS` but this is definitely an improvement as is. I am very very happy to finally learn where `Last` comes from at least :P