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/protos_generator/gen_utils.cc b/protos_generator/gen_utils.cc
new file mode 100644
index 0000000..79dd5a1
--- /dev/null
+++ b/protos_generator/gen_utils.cc
@@ -0,0 +1,152 @@
+// 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_generator/gen_utils.h"
+
+#include <algorithm>
+#include <string>
+#include <vector>
+
+#include "absl/strings/ascii.h"
+
+namespace protos_generator {
+
+namespace protobuf = ::google::protobuf;
+
+void AddEnums(const protobuf::Descriptor* message,
+              std::vector<const protobuf::EnumDescriptor*>* enums) {
+  enums->reserve(enums->size() + message->enum_type_count());
+  for (int i = 0; i < message->enum_type_count(); i++) {
+    enums->push_back(message->enum_type(i));
+  }
+  for (int i = 0; i < message->nested_type_count(); i++) {
+    AddEnums(message->nested_type(i), enums);
+  }
+}
+
+std::vector<const protobuf::EnumDescriptor*> SortedEnums(
+    const protobuf::FileDescriptor* file) {
+  std::vector<const protobuf::EnumDescriptor*> enums;
+  enums.reserve(file->enum_type_count());
+  for (int i = 0; i < file->enum_type_count(); i++) {
+    enums.push_back(file->enum_type(i));
+  }
+  for (int i = 0; i < file->message_type_count(); i++) {
+    AddEnums(file->message_type(i), &enums);
+  }
+  return enums;
+}
+
+void AddMessages(const protobuf::Descriptor* message,
+                 std::vector<const protobuf::Descriptor*>* messages) {
+  messages->push_back(message);
+  for (int i = 0; i < message->nested_type_count(); i++) {
+    AddMessages(message->nested_type(i), messages);
+  }
+}
+
+std::vector<const protobuf::Descriptor*> SortedMessages(
+    const protobuf::FileDescriptor* file) {
+  std::vector<const protobuf::Descriptor*> messages;
+  for (int i = 0; i < file->message_type_count(); i++) {
+    AddMessages(file->message_type(i), &messages);
+  }
+  return messages;
+}
+
+void AddExtensionsFromMessage(
+    const protobuf::Descriptor* message,
+    std::vector<const protobuf::FieldDescriptor*>* exts) {
+  for (int i = 0; i < message->extension_count(); i++) {
+    exts->push_back(message->extension(i));
+  }
+  for (int i = 0; i < message->nested_type_count(); i++) {
+    AddExtensionsFromMessage(message->nested_type(i), exts);
+  }
+}
+
+std::vector<const protobuf::FieldDescriptor*> SortedExtensions(
+    const protobuf::FileDescriptor* file) {
+  const int extension_count = file->extension_count();
+  const int message_type_count = file->message_type_count();
+
+  std::vector<const protobuf::FieldDescriptor*> ret;
+  ret.reserve(extension_count + message_type_count);
+
+  for (int i = 0; i < extension_count; i++) {
+    ret.push_back(file->extension(i));
+  }
+  for (int i = 0; i < message_type_count; i++) {
+    AddExtensionsFromMessage(file->message_type(i), &ret);
+  }
+
+  return ret;
+}
+
+std::vector<const protobuf::FieldDescriptor*> FieldNumberOrder(
+    const protobuf::Descriptor* message) {
+  std::vector<const protobuf::FieldDescriptor*> fields;
+  fields.reserve(message->field_count());
+  for (int i = 0; i < message->field_count(); i++) {
+    fields.push_back(message->field(i));
+  }
+  std::sort(fields.begin(), fields.end(),
+            [](const protobuf::FieldDescriptor* a,
+               const protobuf::FieldDescriptor* b) {
+              return a->number() < b->number();
+            });
+  return fields;
+}
+
+std::string ToCamelCase(const std::string& input, bool lower_first) {
+  bool capitalize_next = !lower_first;
+  std::string result;
+  result.reserve(input.size());
+
+  for (char character : input) {
+    if (character == '_') {
+      capitalize_next = true;
+    } else if (capitalize_next) {
+      result.push_back(absl::ascii_toupper(character));
+      capitalize_next = false;
+    } else {
+      result.push_back(character);
+    }
+  }
+
+  // Lower-case the first letter.
+  if (lower_first && !result.empty()) {
+    result[0] = absl::ascii_tolower(result[0]);
+  }
+
+  return result;
+}
+
+}  // namespace protos_generator