Support nested messages, fix casing. Support test-cases in packages.

betterproto/tests/inputs/casing/casing.proto

@@ -10,6 +10,7 @@ message Test {
   int32 camelCase = 1;
   my_enum snake_case = 2;
   snake_case_message snake_case_message = 3;
+  int32 UPPERCASE = 4;
 }
 
 message snake_case_message {

betterproto/tests/inputs/casing/test_casing.py

@@ -8,6 +8,7 @@ def test_message_attributes():
         message, "snake_case_message"
     ), "snake_case field name is same in python"
     assert hasattr(message, "camel_case"), "CamelCase field is snake_case in python"
+    assert hasattr(message, "uppercase"), "UPPERCASE field is lowercase in python"
 
 
 def test_message_casing():