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/)

cmake.deps/cmake/BuildMsgpack.cmake

@@ -0,0 +1,66 @@
+# BuildMsgpack(CONFIGURE_COMMAND ... BUILD_COMMAND ... INSTALL_COMMAND ...)
+# Reusable function to build msgpack, wraps ExternalProject_Add.
+# Failing to pass a command argument will result in no command being run
+function(BuildMsgpack)
+  cmake_parse_arguments(_msgpack
+    ""
+    ""
+    "CONFIGURE_COMMAND;BUILD_COMMAND;INSTALL_COMMAND"
+    ${ARGN})
+
+  if(NOT _msgpack_CONFIGURE_COMMAND AND NOT _msgpack_BUILD_COMMAND
+       AND NOT _msgpack_INSTALL_COMMAND)
+    message(FATAL_ERROR "Must pass at least one of CONFIGURE_COMMAND, BUILD_COMMAND, INSTALL_COMMAND")
+  endif()
+
+  ExternalProject_Add(msgpack
+    PREFIX ${DEPS_BUILD_DIR}
+    URL ${MSGPACK_URL}
+    DOWNLOAD_DIR ${DEPS_DOWNLOAD_DIR}/msgpack
+    DOWNLOAD_COMMAND ${CMAKE_COMMAND}
+      -DPREFIX=${DEPS_BUILD_DIR}
+      -DDOWNLOAD_DIR=${DEPS_DOWNLOAD_DIR}/msgpack
+      -DURL=${MSGPACK_URL}
+      -DEXPECTED_SHA256=${MSGPACK_SHA256}
+      -DTARGET=msgpack
+      -DUSE_EXISTING_SRC_DIR=${USE_EXISTING_SRC_DIR}
+      -P ${CMAKE_CURRENT_SOURCE_DIR}/cmake/DownloadAndExtractFile.cmake
+    CONFIGURE_COMMAND "${_msgpack_CONFIGURE_COMMAND}"
+    BUILD_COMMAND "${_msgpack_BUILD_COMMAND}"
+    INSTALL_COMMAND "${_msgpack_INSTALL_COMMAND}"
+    LIST_SEPARATOR |)
+endfunction()
+
+set(MSGPACK_CONFIGURE_COMMAND ${CMAKE_COMMAND} ${DEPS_BUILD_DIR}/src/msgpack
+  -DMSGPACK_BUILD_TESTS=OFF
+  -DMSGPACK_BUILD_EXAMPLES=OFF
+  -DCMAKE_INSTALL_PREFIX=${DEPS_INSTALL_DIR}
+  -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
+  -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
+  -DCMAKE_OSX_ARCHITECTURES=${CMAKE_OSX_ARCHITECTURES_ALT_SEP}
+  "-DCMAKE_C_FLAGS:STRING=${CMAKE_C_COMPILER_ARG1} -fPIC"
+  -DCMAKE_GENERATOR=${CMAKE_GENERATOR})
+
+set(MSGPACK_BUILD_COMMAND ${CMAKE_COMMAND} --build . --config ${CMAKE_BUILD_TYPE})
+set(MSGPACK_INSTALL_COMMAND ${CMAKE_COMMAND} --build . --target install --config ${CMAKE_BUILD_TYPE})
+
+if(MSVC)
+  # Same as Unix without fPIC
+  set(MSGPACK_CONFIGURE_COMMAND ${CMAKE_COMMAND} ${DEPS_BUILD_DIR}/src/msgpack
+    -DMSGPACK_BUILD_TESTS=OFF
+    -DMSGPACK_BUILD_EXAMPLES=OFF
+    -DCMAKE_INSTALL_PREFIX=${DEPS_INSTALL_DIR}
+    -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
+    -DCMAKE_GENERATOR_PLATFORM=${CMAKE_GENERATOR_PLATFORM}
+    "-DCMAKE_C_FLAGS:STRING=${CMAKE_C_COMPILER_ARG1}"
+    -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
+    # Make sure we use the same generator, otherwise we may
+    # accidentally end up using different MSVC runtimes
+    -DCMAKE_GENERATOR=${CMAKE_GENERATOR})
+endif()
+
+BuildMsgpack(CONFIGURE_COMMAND ${MSGPACK_CONFIGURE_COMMAND}
+  BUILD_COMMAND ${MSGPACK_BUILD_COMMAND}
+  INSTALL_COMMAND ${MSGPACK_INSTALL_COMMAND})
+
+list(APPEND THIRD_PARTY_DEPS msgpack)