bpo-35181: Correct importlib documentation for some module attributes (GH-15190)

@ericsnowcurrently This PR will change the following:

In the library documentation importlib.rst:

- `module.__package__` can be `module.__name__` for packages;
- `spec.parent` can be `spec.__name__` for packages;
- `spec.loader` is not `None` for namespaces packages.

In the language documentation import.rst:

- `spec.loader` is not `None` for namespace packages.

Automerge-Triggered-By: GH:warsaw

Doc/library/importlib.rst

@@ -438,8 +438,9 @@ ABC hierarchy::
             package. This attribute is not set on modules.
 
         - :attr:`__package__`
-            The parent package for the module/package. If the module is
-            top-level then it has a value of the empty string. The
+            The fully-qualified name of the package under which the module was
+            loaded as a submodule (or the empty string for top-level modules).
+            For packages, it is the same as :attr:`__name__`.  The
             :func:`importlib.util.module_for_loader` decorator can handle the
             details for :attr:`__package__`.
 
@@ -1347,8 +1348,8 @@ find and load modules.
 
    (``__loader__``)
 
-   The loader to use for loading.  For namespace packages this should be
-   set to ``None``.
+   The :term:`Loader <loader>` that should be used when loading
+   the module.  :term:`Finders <finder>` should always set this.
 
    .. attribute:: origin
 
@@ -1381,8 +1382,9 @@ find and load modules.
 
    (``__package__``)
 
-   (Read-only) Fully-qualified name of the package to which the module
-   belongs as a submodule (or ``None``).
+   (Read-only) The fully-qualified name of the package under which the module
+   should be loaded as a submodule (or the empty string for top-level modules).
+   For packages, it is the same as :attr:`__name__`.
 
    .. attribute:: has_location
 

Doc/reference/import.rst

@@ -857,9 +857,8 @@ module.  ``find_spec()`` returns a fully populated spec for the module.
 This spec will always have "loader" set (with one exception).
 
 To indicate to the import machinery that the spec represents a namespace
-:term:`portion`, the path entry finder sets "loader" on the spec to
-``None`` and "submodule_search_locations" to a list containing the
-portion.
+:term:`portion`, the path entry finder sets "submodule_search_locations" to
+a list containing the portion.
 
 .. versionchanged:: 3.4
    :meth:`~importlib.abc.PathEntryFinder.find_spec` replaced
@@ -875,18 +874,7 @@ portion.
    :meth:`~importlib.abc.PathEntryFinder.find_loader` takes one argument, the
    fully qualified name of the module being imported.  ``find_loader()``
    returns a 2-tuple where the first item is the loader and the second item
-   is a namespace :term:`portion`.  When the first item (i.e. the loader) is
-   ``None``, this means that while the path entry finder does not have a
-   loader for the named module, it knows that the path entry contributes to
-   a namespace portion for the named module.  This will almost always be the
-   case where Python is asked to import a namespace package that has no
-   physical presence on the file system.  When a path entry finder returns
-   ``None`` for the loader, the second item of the 2-tuple return value must
-   be a sequence, although it can be empty.
-
-   If ``find_loader()`` returns a non-``None`` loader value, the portion is
-   ignored and the loader is returned from the path based finder, terminating
-   the search through the path entries.
+   is a namespace :term:`portion`.
 
    For backwards compatibility with other implementations of the import
    protocol, many path entry finders also support the same,