Make plugin use betterproto generated classes internally

This means the betterproto plugin no longer needs to depend durectly on
protobuf.

This requires a small runtime hack to monkey patch some google types to
get around the fact that the compiler uses proto2, but betterproto
expects proto3.

Also:
- regenerate google.protobuf package
- fix a regex bug in the logic for determining whether to use a google
  wrapper type.
- fix a bug causing comments to get mixed up when multiple proto files
  generate code into a single python module

src/betterproto/plugin/main.py

@@ -3,9 +3,13 @@
 import os
 import sys
 
-from google.protobuf.compiler import plugin_pb2 as plugin
+from betterproto.lib.google.protobuf.compiler import (
+    CodeGeneratorRequest,
+    CodeGeneratorResponse,
+)
 
 from betterproto.plugin.parser import generate_code
+from betterproto.plugin.models import monkey_patch_oneof_index
 
 
 def main() -> None:
@@ -13,16 +17,19 @@ def main() -> None:
     # Read request message from stdin
     data = sys.stdin.buffer.read()
 
+    # Apply Work around for proto2/3 difference in protoc messages
+    monkey_patch_oneof_index()
+
     # Parse request
-    request = plugin.CodeGeneratorRequest()
-    request.ParseFromString(data)
+    request = CodeGeneratorRequest()
+    request.parse(data)
 
     dump_file = os.getenv("BETTERPROTO_DUMP")
     if dump_file:
         dump_request(dump_file, request)
 
     # Create response
-    response = plugin.CodeGeneratorResponse()
+    response = CodeGeneratorResponse()
 
     # Generate code
     generate_code(request, response)
@@ -34,7 +41,7 @@ def main() -> None:
     sys.stdout.buffer.write(output)
 
 
-def dump_request(dump_file: str, request: plugin.CodeGeneratorRequest) -> None:
+def dump_request(dump_file: str, request: CodeGeneratorRequest) -> None:
     """
     For developers: Supports running plugin.py standalone so its possible to debug it.
     Run protoc (or generate.py) with BETTERPROTO_DUMP="yourfile.bin" to write the request to a file.