Print InspectError traceback in stubgen walk_packages when verbose is specified (#18224)

This change modifies `walk_packages` such that the full `ImporError`
traceback is printed when a module cannot be imported. The goal is to
provide the user with more context to debug the error.

I implemented this change by mirroring existing behavior in
`find_module_paths_using_imports`:
https://github.com/python/mypy/blob/9405bfd9205ea369c11150907764fa46c03cb1f7/mypy/stubgen.py#L1522-L1529

Author: gareth-cross

mypy/stubgen.py

@@ -1524,7 +1524,7 @@ def find_module_paths_using_imports(
             except CantImport as e:
                 tb = traceback.format_exc()
                 if verbose:
-                    sys.stdout.write(tb)
+                    sys.stderr.write(tb)
                 if not quiet:
                     report_missing(mod, e.message, tb)
                 continue

mypy/stubutil.py

@@ -5,6 +5,7 @@
 import os.path
 import re
 import sys
+import traceback
 from abc import abstractmethod
 from collections import defaultdict
 from contextlib import contextmanager
@@ -70,6 +71,9 @@ def walk_packages(
         try:
             prop = inspect.get_package_properties(package_name)
         except InspectError:
+            if verbose:
+                tb = traceback.format_exc()
+                sys.stderr.write(tb)
             report_missing(package_name)
             continue
         yield prop.name