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

betterproto/compile/naming.py

@@ -0,0 +1,13 @@
+from betterproto import casing
+
+
+def pythonize_class_name(name):
+    return casing.pascal_case(name)
+
+
+def pythonize_field_name(name: str):
+    return casing.safe_snake_case(name)
+
+
+def pythonize_method_name(name: str):
+    return casing.safe_snake_case(name)