consumption of method_signature annotation doesn't account for dot-notation access of nested fields for a required field

The following code:
https://github.com/googleapis/gapic-generator-php/blob/277c7036edbcd245fc38cd3a97a761807af05188/src/Generation/BuildMethodFragmentGenerator.php#L135-L141

Does not account for a signature like `"foo.a,foo.b"` where `foo` is a `google.api.field_behavior = REQUIRED` field. The presence of `foo.a` should be sufficient.

Furthermore, we need to make sure that `build` fragment generation can handle a dot-notation signature!