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
diff --git a/upbc/code_generator_request.h b/upbc/code_generator_request.h
new file mode 100644
index 0000000..3218c08
--- /dev/null
+++ b/upbc/code_generator_request.h
@@ -0,0 +1,55 @@
+// 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.
+
+#ifndef UPBC_CODE_GENERATOR_REQUEST_H_
+#define UPBC_CODE_GENERATOR_REQUEST_H_
+
+#include "upb/mem/arena.h"
+#include "upb/reflection/def.h"
+#include "upbc/code_generator_request.upb.h"
+
+// Must be last.
+#include "upb/port/def.inc"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+upbc_CodeGeneratorRequest* upbc_MakeCodeGeneratorRequest(
+    struct google_protobuf_compiler_CodeGeneratorRequest* request, upb_Arena* a,
+    upb_Status* s);
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+#include "upb/port/undef.inc"
+
+#endif /* UPBC_CODE_GENERATOR_REQUEST_H_ */