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/)

test/cmakeconfig/paths.lua.in

@@ -0,0 +1,24 @@
+local module = {}
+
+module.include_paths = {}
+for p in ("${TEST_INCLUDE_DIRS}" .. ";"):gmatch("[^;]+") do
+  table.insert(module.include_paths, p)
+end
+
+module.test_build_dir = "${CMAKE_BINARY_DIR}"
+module.test_include_path = module.test_build_dir .. "/test/includes/post"
+module.test_libnvim_path = "${TEST_LIBNVIM_PATH}"
+module.test_source_path = "${CMAKE_SOURCE_DIR}"
+module.test_lua_prg = "${LUA_PRG}"
+module.test_luajit_prg = ""
+if module.test_luajit_prg == '' then
+  if module.test_lua_prg:sub(-6) == 'luajit' then
+    module.test_luajit_prg = module.test_lua_prg
+  else
+    module.test_luajit_prg = nil
+  end
+end
+table.insert(module.include_paths, "${CMAKE_BINARY_DIR}/include")
+table.insert(module.include_paths, "${CMAKE_BINARY_DIR}/src/nvim/auto")
+
+return module