Consider only top-level paths in switch

[htdvisser]

Summary:

This PR addresses yesterday's discussion about the generated code not being able to properly handle nested paths. This PR should be considered as a way to move forward with MVP until we have a consensus about the goals and implementation of feature/rewrite.

Changes:

• SetFields now only ranges over the top level paths of the field path
• If a field is a message, the sub-field-paths are filtered (so for field a, a.b,a.c is converted to b,c) and we delegate to the sub-message's SetFields

[rvolosatovs]

This is already implemented in https://github.com/TheThingsIndustries/protoc-gen-fieldmask/pull/1 without the need for helper functions