Add --include-vendored flag and warning for skipped vendor modules

[veeceey]

Summary

This PR addresses the issue where stubgen silently skips directories named 'vendor', 'vendored', '_vendor', etc., even when they are legitimate user packages rather than vendored third-party code.

Changes

• Added --include-vendored flag to allow generating stubs for vendored packages
• Modified remove_blacklisted_modules() to print a warning message when skipping vendored modules (unless --quiet is specified)
• Updated Options class to include the include_vendored parameter

Behavior

Before

Running stubgen a on a project with structure:

a/
  __init__.py
  vendor/
    __init__.py

Would silently skip the vendor directory with no indication to the user.

After

Running stubgen a will now print:

Skipping vendored module: a.vendor

To generate stubs for the vendor directory, users can use:

stubgen --include-vendored a

Or suppress the warning with:

stubgen -q a

Test Plan

1. Create test directory structure with a 'vendor' package
2. Run stubgen without flags - verify warning is printed and vendor is skipped
3. Run stubgen --include-vendored - verify vendor stubs are generated
4. Run stubgen -q - verify no warning is printed

Fixes #9599