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

tests/inputs/oneof/oneof.proto

@@ -2,7 +2,15 @@ syntax = "proto3";
 
 message Test {
   oneof foo {
-    int32 count = 1;
-    string name = 2;
+    int32 pitied = 1;
+    string pitier = 2;
+  }
+
+  int32 just_a_regular_field = 3;
+
+  oneof bar {
+    int32 drinks = 11;
+    string bar_name = 12;
   }
 }
+