Respect the `ruby_package` option when generating twirp files

[darronschall]

protoc supports the ruby_package option in proto files to specify the module of the generated ruby code (see protocolbuffers/protobuf#4627).

The twirp generator should also respect the ruby_package option if present, and prefer that over the proto file's package.

To implement this, we'll likely want to build up a type mapping for the proto file and all of the proto file imports (where the imports might also leverage ruby_package), rather than naive approach we took in our initial convert_to_ruby_type at https://github.com/collectiveidea/protoc-gen-twirp_ruby/blob/v1.0.0/lib/twirp/protoc_plugin/code_generator.rb#L145