Print InspectError traceback in stubgen walk_packages when verbose is specified

[gareth-cross]

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:

mypy/mypy/stubgen.py

Lines 1522 to 1529 in 9405bfd

	try:
	result = find_module_path_and_all_py3(inspect, mod, verbose)
	except CantImport as e:
	tb = traceback.format_exc()
	if verbose:
	sys.stdout.write(tb)
	if not quiet:
	report_missing(mod, e.message, tb)

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅