vendor: alias missing vendored submodules when DEBUNDLED

[gcomneno]

This updates pip._vendor.__init__ to alias additional vendored submodules under DEBUNDLED.

These submodules are imported from pip._internal but were not previously included in the vendored() alias list, which can impact downstream redistributors running pip in a debundled configuration.

No behavioral change for default (bundled) installs since this code path is only active when DEBUNDLED = True.

Tests:

• python -m pytest -q tests/unit/test_vendor_aliases.py

[gcomneno]

Fixes debundled-mode vendor aliasing: add missing vendored() entries for modules imported from pip._vendor (e.g. requests.adapters, packaging.tags, urllib3.*) and add a unit test to prevent regressions. This matters for downstream redistributors running pip in debundled setups.

[gcomneno]

CI is green (all checks passed). The earlier macOS 3.13 failure was flaky (test_utils_retry timing). Ready for review.