Ensure all pip._vendor.* modules are mapped to debundled correspondences (#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 https://github.com/pypa/pip/issues/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