Make build metadata visible to packages

[Herringway]

It would be nice for a package to be able to access the same data available via dub describe's output at compile time. This could enable a package to be able to report its own version as well as those of its dependencies, for example. This could be achieved through an autogenerated module, like dub.build or something along those lines. While it is possible to achieve this through a pre-build step that processes dub describe's output, it can be pretty unreliable to do it this way.