message: api_proto_plugin: create visitor per file
  Create visitor.Visitor per proto file to get correct information from TypeWhisperer.
  This will make it easier to track imports in protoxform as well.
  Risk Level: Low
  Testing: CI
  Fixes
diff: @@ -17,8 +17,8 @@ OutputDescriptor = namedtuple(
  # Output files are generated alongside their corresponding input .proto,
  # with the output_suffix appended.
  'output_suffix',
  - # The visitor is a function to create a visitor.Visitor defining the business
  - # logic of the plugin for the specific output descriptor.
  + # The visitor factory is a function to create a visitor.Visitor defining
  + # the business logic of the plugin for the specific output descriptor.
  'visitor_factory',
  # FileDescriptorProto transformer; this is applied to the input
  # before any output generation.
  