build: rename build-related dirs

Problem:
Dirs "config", "packaging", and "third-party" are all closely related
but this is not obvious from the layout. This adds friction for new
contributors.

Solution:
- rename config/ to cmake.config/
- rename test/config/ to test/cmakeconfig/ because it is used in Lua
  tests: require('test.cmakeconfig.paths').
- rename packaging/ to cmake.packaging/
- rename third-party/ to cmake.deps/ (parallel with .deps/)

README.md

@@ -81,8 +81,10 @@ Project layout
 --------------
 
     ├─ ci/              build automation
-    ├─ cmake/           build scripts
-    ├─ runtime/         user plugins/docs
+    ├─ cmake/           CMake utils
+    ├─ cmake.config/    CMake defines
+    ├─ cmake.deps/      subproject to fetch and build dependencies (optional)
+    ├─ runtime/         plugins and docs
     ├─ src/nvim/        application source code (see src/nvim/README.md)
     │  ├─ api/          API subsystem
     │  ├─ eval/         VimL subsystem
@@ -93,7 +95,6 @@ Project layout
     │  ├─ msgpack_rpc/  RPC subsystem
     │  ├─ os/           low-level platform code
     │  └─ tui/          built-in UI
-    ├─ third-party/     CMake subproject to build dependencies
     └─ test/            tests (see test/README.md)
 
 License