Source-location aware parser for package descriptions

It would be nice if Cabal included a parser for `.cabal` files that remembers line and column numbers for parsed entities. This could then be used by tools like packdeps to give the source location for an outdated dependency, and in turn used in text editors to annotate lines with information from packdeps when editing a .cabal file.

More broadly, it would be nice to have an exact-printer for the relevant AST, to make it possible to write tools that process .cabal files programmatically without messing up their formatting.