Add CURRENT_PACKAGE_VERSION or similar to cabal-macros.h

Given that cabal-macros.h defines VERSION_[pkg] for every dep, it seems strange that we can't access something as simple as the current package version, unless of course I'm missing something.

I assume this would be a very easy and non-breaking change, and am happy to provide a PR. However there may be dragons, or it may already exist, so asking first!

Start by inspecting cabal-macros.h and how its VERSION_[pkg] macros are defined. Confirm whether a current-package version macro already exists, then determine how to expose the requested value without breaking existing macros; done means the current package version is accessible through the header.

Written by the indexing model from the issue text.