autodoc_type_aliases does not support dotted name

[tk0miya]

Describe the bug
autodoc_type_aliases does not support dotted name

To Reproduce

# helloworld.py
from __future__ import annotations  # important!
import struct

def consume_struct(_: struct.Struct) -> None:
    pass

# conf.py
autodoc_type_aliases = {
    'struct.Struct': 'struct.Struct',
}

The autodoc_type_aliases entry does not work as expected.
(refs: #8315)

Expected behavior
Supporting

Your project
No

Screenshots
No

Environment info

• OS: Mac
• Python version: 3.8.2
• Sphinx version: HEAD of 3.x
• Sphinx extensions: sphinx.ext.autodoc
• Extra tools: No

Additional context
No

[shimizukawa]

quick hacking...

>>> class ModuleProxy:
...     def __init__(self, module_name, mapper):
...         self._mod = __import__(module_name)
...         self._map = mapper
...     def __getattr__(self, key):
...         if key in self._map:
...             return self._map[key]
...         return getattr(self._mod, key)
...
>>> autodoc_type_aliases = {
...    'struct': ModuleProxy('struct', {'Struct': 'struct.Struct'}),
... }
>>>
>>> from __future__ import annotations  # important!
>>> import struct
>>>
>>> def consume_struct(_: struct.Struct) -> struct.error:
...     pass
...
>>> typing.get_type_hints(consume_struct, localns=magical)
{'_': ForwardRef('struct.Struct'), 'return': <class 'struct.error'>}

... this can be applied if Sphinx knows that struct is a module.