Formalize sub-directories as library targets

[karnkaul]

Dependency graph:

ext/fmt    ext/glm   ext/stb
  ↓           ↓        ↓
  ----------------------
   ↓
  util   ext/vma
   ↓        ↓
   ----------
       ↓
       vk    ext/glfw
       ↓        ↓
       ----------
         ↓
        glfw   ext/dear_imgui
         ↓           ↓
         -------------
             ↓
           render             ext/djson
           ↓   ↓                  ↓
   --------     -------------------
   ↓                       ↓
 engine                  scene
   ↓                      ↓ ↓
   ------------------------ -------
            ↓                     ↓
         context                editor