Reorganize upb file structure

This change moves almost everything in the `upb/` directory up one level, so
that for example `upb/upb/generated_code_support.h` becomes just
`upb/generated_code_support.h`. The only exceptions I made to this were that I
left `upb/cmake` and `upb/BUILD` where they are, mostly because that avoids
conflict with other files and the current locations seem reasonable for now.

The `python/` directory is a little bit of a challenge because we had to merge
the existing directory there with `upb/python/`. I made `upb/python/BUILD` into
the BUILD file for the merged directory, and it effectively loads the contents
of the other BUILD file via `python/build_targets.bzl`, but I plan to clean
this up soon.

PiperOrigin-RevId: 568651768

protos/protos_internal_test.cc

diff --git a/protos/protos_internal_test.cc b/protos/protos_internal_test.cc
new file mode 100644
index 0000000..a173665
--- /dev/null
+++ b/protos/protos_internal_test.cc
@@ -0,0 +1,60 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2023 Google LLC.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google LLC nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "protos/protos_internal.h"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+#include "protos_generator/tests/test_model.upb.h"
+#include "protos_generator/tests/test_model.upb.proto.h"
+#include "upb/mem/arena.h"
+
+namespace protos::testing {
+namespace {
+using ::protos_generator::test::protos::TestModel;
+
+TEST(CppGeneratedCode, InternalMoveMessage) {
+  // Generate message (simulating message created in another VM/language)
+  upb_Arena* source_arena = upb_Arena_New();
+  protos_generator_test_TestModel* message =
+      protos_generator_test_TestModel_new(source_arena);
+  ASSERT_NE(message, nullptr);
+  protos_generator_test_TestModel_set_int_value_with_default(message, 123);
+
+  // Move ownership.
+  TestModel model =
+      protos::internal::MoveMessage<TestModel>(message, source_arena);
+  // Now that we have moved ownership, free original arena.
+  upb_Arena_Free(source_arena);
+  EXPECT_EQ(model.int_value_with_default(), 123);
+}
+
+}  // namespace
+}  // namespace protos::testing