api_proto_plugin: create visitor per file (envoyproxy#9267)

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 envoyproxy#9266

Signed-off-by: Lizan Zhou <lizan@tetrate.io>

tools/api_proto_plugin/plugin.py

@@ -17,8 +17,8 @@
         # 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.