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.

cmake/FindPajladaSerialize.cmake

@@ -1,14 +0,0 @@
-include(FindPackageHandleStandardArgs)
-
-find_path(PajladaSerialize_INCLUDE_DIR pajlada/serialize.hpp HINTS ${CMAKE_SOURCE_DIR}/lib/serialize/include)
-
-find_package_handle_standard_args(PajladaSerialize DEFAULT_MSG PajladaSerialize_INCLUDE_DIR)
-
-if (PajladaSerialize_FOUND)
-    add_library(Pajlada::Serialize INTERFACE IMPORTED)
-    set_target_properties(Pajlada::Serialize PROPERTIES
-            INTERFACE_INCLUDE_DIRECTORIES "${PajladaSerialize_INCLUDE_DIR}"
-            )
-endif ()
-
-mark_as_advanced(PajladaSerialize_INCLUDE_DIR)

cmake/FindPajladaSettings.cmake

@@ -1,16 +0,0 @@
-include(FindPackageHandleStandardArgs)
-
-find_path(PajladaSettings_INCLUDE_DIR pajlada/settings.hpp)
-find_library(PajladaSettings_LIBRARY PajladaSettings)
-
-find_package_handle_standard_args(PajladaSettings DEFAULT_MSG PajladaSettings_INCLUDE_DIR PajladaSettings_LIBRARY)
-
-if (PajladaSettings_FOUND)
-    add_library(Pajlada::Settings INTERFACE IMPORTED)
-    set_target_properties(Pajlada::Settings PROPERTIES
-            INTERFACE_INCLUDE_DIRECTORIES "${PajladaSettings_INCLUDE_DIR}"
-            INTERFACE_LINK_LIBRARIES "${PajladaSettings_LIBRARY}"
-            )
-endif ()
-
-mark_as_advanced(PajladaSettings_INCLUDE_DIR PajladaSettings_LIBRARY)

cmake/FindPajladaSignals.cmake

@@ -1,14 +0,0 @@
-include(FindPackageHandleStandardArgs)
-
-find_path(PajladaSignals_INCLUDE_DIR pajlada/signals/signal.hpp HINTS ${CMAKE_SOURCE_DIR}/lib/signals/include)
-
-find_package_handle_standard_args(PajladaSignals DEFAULT_MSG PajladaSignals_INCLUDE_DIR)
-
-if (PajladaSignals_FOUND)
-    add_library(Pajlada::Signals INTERFACE IMPORTED)
-    set_target_properties(Pajlada::Signals PROPERTIES
-            INTERFACE_INCLUDE_DIRECTORIES "${PajladaSignals_INCLUDE_DIR}"
-            )
-endif ()
-
-mark_as_advanced(PajladaSignals_INCLUDE_DIR)

cmake/FindRapidJSON.cmake

@@ -1,14 +0,0 @@
-include(FindPackageHandleStandardArgs)
-
-find_path(RapidJSON_INCLUDE_DIR rapidjson/rapidjson.h HINTS ${CMAKE_SOURCE_DIR}/lib/rapidjson/include)
-
-find_package_handle_standard_args(RapidJSON DEFAULT_MSG RapidJSON_INCLUDE_DIR)
-
-if (RapidJSON_FOUND)
-    add_library(RapidJSON::RapidJSON INTERFACE IMPORTED)
-    set_target_properties(RapidJSON::RapidJSON PROPERTIES
-            INTERFACE_INCLUDE_DIRECTORIES "${RapidJSON_INCLUDE_DIR}"
-            )
-endif ()
-
-mark_as_advanced(RapidJSON_INCLUDE_DIR)