Leave tool/pigeon_codegen out of the root analysis

It is its own package with its own pubspec, which a fresh checkout's
root pub get does not resolve, so CI's analyze reported its imports as
missing.

analysis_options.yaml

@@ -19,6 +19,8 @@ analyzer:
     - macos/**
     - linux/**
     - pigeons/**
+    # Its own package, with its own pubspec; the root does not resolve it.
+    - tool/pigeon_codegen/**
     - third_party/**
 
 plugins: