Distinguish human created and machine derived data in `.cabal` files

One example of where this comes up is with `other-extensions`, which can sometimes be machine-derived. Quoting @ezyang from [here](https://github.com/haskell/cabal/issues/1562#issuecomment-243694149):

> I am very sympathetic to your concern, but unfortunately having it be implicit would sort of miss the point, because the dependency solver wants to make decisions about what packages to choose without having to actually look at the Haskell files (it's got only the Cabal file, not the full tarball.)
>
> Maybe the moral of the story is that Cabal metadata should be divided into two parts: one that is hand-written, and one that is machine generated. When you upload, you upload both parts (so you have to be able to generate the machine bits.)