cmake: use FetchContent for some dependencies (#6560)

this transition is _not_ complete, but we use FetchContent for
RapidJSON, PajladaSerialize, PajladaSettings, and PajladaSignals.

In the future, we can move towards providing as many of our dependencies
as possible through conan & vcpkg so submodule-less builds are possible.

CMakeLists.txt

@@ -2,6 +2,7 @@ cmake_minimum_required(VERSION 3.15)
 cmake_policy(SET CMP0087 NEW) # evaluates generator expressions in `install(CODE/SCRIPT)`
 cmake_policy(SET CMP0091 NEW) # select MSVC runtime library through `CMAKE_MSVC_RUNTIME_LIBRARY`
 include(FeatureSummary)
+include(FetchContent)
 
 list(APPEND CMAKE_MODULE_PATH
     "${CMAKE_SOURCE_DIR}/cmake"
@@ -195,8 +196,6 @@ if (BUILD_WITH_QTKEYCHAIN)
     endif()
 endif()
 
-find_package(RapidJSON REQUIRED)
-
 find_package(Websocketpp REQUIRED)
 
 if (BUILD_TESTS)
@@ -224,23 +223,38 @@ if (BUILD_BENCHMARKS)
     find_package(benchmark REQUIRED)
 endif ()
 
-find_package(PajladaSerialize REQUIRED)
-find_package(PajladaSignals REQUIRED)
+FetchContent_Declare(
+    rapidjson
+    SOURCE_DIR ${CMAKE_CURRENT_LIST_DIR}/lib/rapidjson
+    EXCLUDE_FROM_ALL
+    FIND_PACKAGE_ARGS NAMES RapidJSON
+)
+set(RAPIDJSON_BUILD_EXAMPLES Off CACHE INTERNAL "")
+set(RAPIDJSON_BUILD_TESTS Off CACHE INTERNAL "")
+
+FetchContent_Declare(
+    PajladaSignals
+    SOURCE_DIR ${CMAKE_CURRENT_LIST_DIR}/lib/signals
+    EXCLUDE_FROM_ALL
+)
+FetchContent_Declare(
+    PajladaSerialize
+    SOURCE_DIR ${CMAKE_CURRENT_LIST_DIR}/lib/serialize
+    EXCLUDE_FROM_ALL
+)
+FetchContent_Declare(
+    PajladaSettings
+    SOURCE_DIR ${CMAKE_CURRENT_LIST_DIR}/lib/settings
+    EXCLUDE_FROM_ALL
+)
+
+FetchContent_MakeAvailable(rapidjson PajladaSignals PajladaSerialize PajladaSettings)
+
 find_package(LRUCache REQUIRED)
 find_package(MagicEnum REQUIRED)
 find_package(Doxygen)
 find_package(BoostCertify REQUIRED)
 
-if (USE_SYSTEM_PAJLADA_SETTINGS)
-    find_package(PajladaSettings REQUIRED)
-else()
-    if (NOT EXISTS "${CMAKE_SOURCE_DIR}/lib/settings/CMakeLists.txt")
-        message(FATAL_ERROR "Submodules probably not loaded, unable to find lib/settings/CMakeLists.txt")
-    endif()
-
-    add_subdirectory("${CMAKE_SOURCE_DIR}/lib/settings" EXCLUDE_FROM_ALL)
-endif()
-
 if (CHATTERINO_PLUGINS)
     set(LUA_INCLUDE_DIRS "${CMAKE_SOURCE_DIR}/lib/lua/src")
     add_subdirectory(lib/lua)