Fix debundled support by add a missing entry for appdirs

[lazka]

pip has started to use the vendored appdirs directly since #7501
but didn't add an alias for the unbundled case.

This adds the missing alias.

---

Before the fix:

$ pip
Traceback (most recent call last):
  File "C:\msys64\mingw64\bin\pip-script.py", line 11, in <module>
    load_entry_point('pip==20.0.2', 'console_scripts', 'pip')()
  File "C:/msys64/mingw64/lib/python3.8/site-packages/pkg_resources/__init__.py", line 490, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "C:/msys64/mingw64/lib/python3.8/site-packages/pkg_resources/__init__.py", line 2853, in load_entry_point
    return ep.load()
  File "C:/msys64/mingw64/lib/python3.8/site-packages/pkg_resources/__init__.py", line 2444, in load
    return self.resolve()
  File "C:/msys64/mingw64/lib/python3.8/site-packages/pkg_resources/__init__.py", line 2450, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "C:/msys64/mingw64/lib/python3.8/site-packages/pip/_internal/cli/main.py", line 10, in <module>
    from pip._internal.cli.autocompletion import autocomplete
  File "C:/msys64/mingw64/lib/python3.8/site-packages/pip/_internal/cli/autocompletion.py", line 9, in <module>
    from pip._internal.cli.main_parser import create_main_parser
  File "C:/msys64/mingw64/lib/python3.8/site-packages/pip/_internal/cli/main_parser.py", line 7, in <module>
    from pip._internal.cli import cmdoptions
  File "C:/msys64/mingw64/lib/python3.8/site-packages/pip/_internal/cli/cmdoptions.py", line 25, in <module>
    from pip._internal.locations import USER_CACHE_DIR, get_src_prefix
  File "C:/msys64/mingw64/lib/python3.8/site-packages/pip/_internal/locations.py", line 19, in <module>
    from pip._internal.utils import appdirs
  File "C:/msys64/mingw64/lib/python3.8/site-packages/pip/_internal/utils/appdirs.py", line 13, in <module>
    from pip._vendor import appdirs as _appdirs
ImportError

[lazka]

I assume this doesn't need a NEWS entry / issue, but I'd be happy to create them if wanted.

[uranusjr]

appdirs was vendored much earlier than #7501 it was used bu another vendored lib), that PR only dropped the duplicated functionality :p

Nonetheless, thanks for catching this!

[chrahunt]

@pradyunsg, do we want to include this in 20.x?

[pradyunsg]

I'm on board. Since we're probably going to be making a release for the PyPy tags anyway, I'll be happy to include this in the release as well. :)

[chrahunt]

@lazka, can you please add a bugfix news entry for this?

[lazka]

@chrahunt done

[pradyunsg]

@chrahunt I don't think this should get a .bugfix entry, since debundling is "semi-supported". I'd prefer that we don't call this out in NEWS at all or, at best, call this out in our vendoring details (i.e. a .vendor suffix).

I'll merge this PR now, since I don't think the discussion on where the NEWS entry should be is a blocker for this PR. I'll file a follow-up PR where we can discuss that. :)

[pradyunsg]

Thanks for the PR @lazka! ^>^