Minor: Align dub.recipe.selection style with dub.recipe.packagerecipe
Minor: Align dub.recipe.selection style with dub.recipe.packagerecipe

During review of the PR that switched PackageRecipe parsing to use Configy,
WebFreak pointed out that having an internal, private structure used for
parsing YAML field was preferable to having it at module level.
This aligns the names and the approach used with the one in PackageRecipe.

Geod24/dub