[build] CMake: simplify source JAR creation and install (#6831)

Bump required minimum CMake version to 3.21.

cmake/modules/CreateSourceJar.cmake

@@ -0,0 +1,34 @@
+macro(add_source_jar target)
+    set(oneValueArgs OUTPUT_NAME OUTPUT_DIR)
+    cmake_parse_arguments(SOURCE_JAR "" "${oneValueArgs}" "BASE_DIRECTORIES" ${ARGN})
+    foreach(base_package_dir ${SOURCE_JAR_BASE_DIRECTORIES})
+        file(GLOB_RECURSE directories LIST_DIRECTORIES true ${base_package_dir}/*.directoriesonly)
+        # Find all packages
+        foreach(directory ${directories})
+            cmake_path(
+                RELATIVE_PATH
+                directory
+                BASE_DIRECTORY ${base_package_dir}
+                OUTPUT_VARIABLE package_name
+            )
+            file(GLOB package_sources ${directory}/*.java)
+            if(package_sources STREQUAL "")
+                continue()
+            endif()
+            # If package sources are scattered across different places, consolidate them under one package
+            list(FIND packages ${package_name} index)
+            if(index EQUAL -1)
+                list(APPEND packages ${package_name})
+            endif()
+            list(APPEND ${package_name} ${package_sources})
+        endforeach()
+    endforeach()
+    set(resources "")
+    foreach(package ${packages})
+        string(APPEND resources "NAMESPACE \"${package}\" ${${package}} ")
+    endforeach()
+    cmake_language(
+        EVAL CODE
+            "add_jar(${target} RESOURCES ${resources} OUTPUT_NAME ${SOURCE_JAR_OUTPUT_NAME} OUTPUT_DIR ${SOURCE_JAR_OUTPUT_DIR})"
+    )
+endmacro()