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/__init__.py

@@ -1172,6 +1172,7 @@ from .lib.google.protobuf import (  # noqa
     BytesValue,
     DoubleValue,
     Duration,
+    EnumValue,
     FloatValue,
     Int32Value,
     Int64Value,
@@ -1238,14 +1239,17 @@ class _WrappedMessage(Message):
 
 def _get_wrapper(proto_type: str) -> Type:
     """Get the wrapper message class for a wrapped type."""
+
+    # TODO: include ListValue and NullValue?
     return {
         TYPE_BOOL: BoolValue,
-        TYPE_INT32: Int32Value,
-        TYPE_UINT32: UInt32Value,
-        TYPE_INT64: Int64Value,
-        TYPE_UINT64: UInt64Value,
-        TYPE_FLOAT: FloatValue,
-        TYPE_DOUBLE: DoubleValue,
-        TYPE_STRING: StringValue,
         TYPE_BYTES: BytesValue,
+        TYPE_DOUBLE: DoubleValue,
+        TYPE_FLOAT: FloatValue,
+        TYPE_ENUM: EnumValue,
+        TYPE_INT32: Int32Value,
+        TYPE_INT64: Int64Value,
+        TYPE_STRING: StringValue,
+        TYPE_UINT32: UInt32Value,
+        TYPE_UINT64: UInt64Value,
     }[proto_type]