Generating PackageInfo_* files for dependencies to retire most usages of CPP.

In order to avoid using CPP and messing with source tooling like formatters and linters, I'd like cabal to provide me with `PackageInfo_*` modules for my dependencies, so that I may use Template Haskell in order to provide different code snippets based on the version of my dependencies.

Basically, a Haskell rendition of `cabal_macros.h`, which could perhaps remain useful for use with FFI, but its CPP would not pollute Haskell source code anymore.