Ensure all pip._vendor.* modules are mapped to debundled corresponden…

…ces (#6113)

With the original `vendored()` implementation and such an initialization sequence:

```
vendored("packaging")
vendored("packaging.version")
```

In `sys.modules`, `pip._vendor.packaging` is correctly connected to the debundled `packaging`, while `pip._vendor.packaging.version` is not, as the latter is `__import__`ed from the existing `pip._vendor.packaging` module. That results in the same issue as #5429 - `pip._vendor.packaging.version.Version` and `packaging.version.Version` cannot be compared.

This patch attempts to fix this issue by skipping `__import__` from the vendored name. This is safe because `vendored()` is called only when `DEBUNDLED = True`, and vendored libraries are already deleted as per [debundling instructions](https://github.com/pypa/pip/blob/master/src/pip/_vendor/README.rst#debundling).

src/pip/_vendor/README.rst

@@ -146,3 +146,6 @@ extra work on your end in order to solve the problems described above.
 6. *(optional)* Update the ``pip_version_check`` logic to use the
  appropriate logic for determining the latest available version of pip and
  prompt the user with the correct upgrade message.
+
+Note that partial debundling is **NOT** supported. You need to prepare wheels
+for all dependencies for successful debundling.

src/pip/_vendor/__init__.py

@@ -30,24 +30,21 @@ def vendored(modulename):
  vendored_name = "{0}.{1}".format(__name__, modulename)
 
  try:
- __import__(vendored_name, globals(), locals(), level=0)
+ __import__(modulename, globals(), locals(), level=0)
  except ImportError:
- try:
- __import__(modulename, globals(), locals(), level=0)
- except ImportError:
- # We can just silently allow import failures to pass here. If we
- # got to this point it means that ``import pip._vendor.whatever``
- # failed and so did ``import whatever``. Since we're importing this
- # upfront in an attempt to alias imports, not erroring here will
- # just mean we get a regular import error whenever pip *actually*
- # tries to import one of these modules to use it, which actually
- # gives us a better error message than we would have otherwise
- # gotten.
- pass
- else:
- sys.modules[vendored_name] = sys.modules[modulename]
- base, head = vendored_name.rsplit(".", 1)
- setattr(sys.modules[base], head, sys.modules[modulename])
+ # We can just silently allow import failures to pass here. If we
+ # got to this point it means that ``import pip._vendor.whatever``
+ # failed and so did ``import whatever``. Since we're importing this
+ # upfront in an attempt to alias imports, not erroring here will
+ # just mean we get a regular import error whenever pip *actually*
+ # tries to import one of these modules to use it, which actually
+ # gives us a better error message than we would have otherwise
+ # gotten.
+ pass
+ else:
+ sys.modules[vendored_name] = sys.modules[modulename]
+ base, head = vendored_name.rsplit(".", 1)
+ setattr(sys.modules[base], head, sys.modules[modulename])
 
 
 # If we're operating in a debundled setup, then we want to go ahead and trigger