Copy the flutter_gpu package into the engine artifacts. (#51365)

Part of flutter/flutter#131711.

Author: bdero

build/archives/BUILD.gn

@@ -87,6 +87,10 @@ if (build_engine_artifacts && !is_android) {
           source = "//flutter/impeller/compiler/shader_lib"
           destination = "shader_lib"
         },
+        {
+          source = "//flutter/lib/gpu"
+          destination = "flutter_gpu"
+        },
         {
           source = "$root_out_dir/${lib_prefix}path_ops$dll"
           destination = "${lib_prefix}path_ops$dll"