Fixes sphinx-doc#13934

This commit ensures that the `__annotations__` dict of a class is
updated only with type comment-derived annotations for direct members;
the type comment-derived annotations for parent classes are added to
those parent classes own `__annotations__` dicts.  The inherited
member annotations are still found by `typing.get_type_hints`.

This commit also improves the efficiency of incorporating the
annotations found by the ModuleAnalyzer.  Previously, each call to
_load_object_by_name for a data member or attribute would result in
separately iterating over all attributes (including within classes)
found by ModuleAnalyzer for the containing module, and in the case of
classes, the containing modules of all parent classes.  For modules
with many members this cost could likely be significant.  With this
commit, at least for the purpose of updating `__annotations__` dicts,
each module's attributes only processed a single time in total.

# Conflicts:
#	sphinx/ext/autodoc/importer.py