Pull in all subpackages in the main package

Hello,

It is an intuitive behavior for a project like DSFML to pull in all subpackages if you specify the main package as a dependency. Right now putting `dsfml` as a dependency in your DUB package does nothing of worth – one must use specific subpackages. My proposal is that the main package should pull in all subpackages, which is the usual behavior of similar DUB packages.