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

README.md

@@ -443,9 +443,9 @@ Assuming your `google.protobuf` source files (included with all releases of `pro
 
 ```sh
 protoc \
-    --plugin=protoc-gen-custom=betterproto/plugin.py \
+    --plugin=protoc-gen-custom=src/betterproto/plugin/main.py \
     --custom_opt=INCLUDE_GOOGLE \
-    --custom_out=betterproto/lib \
+    --custom_out=src/betterproto/lib \
     -I /usr/local/include/ \
     /usr/local/include/google/protobuf/*.proto
 ```