task for vendoring pluggy

[nicoddemus]

Next time we vendor pluggy, we should create an invoke task to do that automatically; also, the same task should update all internal links in the docs to point to the new location, for example:

same arguments as ``before`` but also a :py:class:`_CallOutcome`` object

To:

same arguments as ``before`` but also a :py:class:`_CallOutcome <_pytest.vendored_packages.pluggy._CallOutcome>` object

[The-Compiler]

FWIW pip has https://github.com/pypa/pip/blob/master/tasks/vendoring/__init__.py - though they also do import rewriting and patching there.

[nicoddemus]

xref: #2473