Move foreign code from global scope to dub.internal. ...

Avoids conflicting with the actual dub packages when working with dub as a library. Also, even if just by convention, it ensures that dependent code doesn't rely on this code as a public API.

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.

Configy: Replace parseField with parseAs ...

This removes the need to keep the representation around and have
it stored in a field of the enclosing struct.

Use Configy to read dub.selections.json

Various code simplification around `dub.selections.json` (#2311) ...

* Deprecate unused Json constructor

* Trivial: Fix import order in dub.project

* Simplify DependencyResolver filtering

DependencyResolver is only instantiated from one place, and never re-used.
It makes more sense to set those variables in the constructor,
and avoid repeatedly calling a single function.

* Move file format to dub.selections.json to its own struct

Our goal is to replace the handcrafted JSON code with configy.