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.