Fix #8541: autodoc_type_aliases doesn't work for the instance attrs

So far, autodoc obtains type annotations of instance attributes by
ModuleAnalyzer directly.  As a result, autodoc_type_aliases are ignored
for these variables.

This goes to merge type annotations from the class itself and
ModuleAnalyzer's, and get type annotations using `typing.get_type_hints()`
to apply autodoc_type_aliases.

tests/roots/test-ext-autodoc/target/annotations.py

@@ -7,6 +7,9 @@ myint = int
 #: docstring
 variable: myint
 
+#: docstring
+variable2 = None  # type: myint
+
 
 def sum(x: myint, y: myint) -> myint:
     """docstring"""
@@ -32,4 +35,7 @@ class Foo:
     """docstring"""
 
     #: docstring
-    attr: myint
+    attr1: myint
+
+    def __init__(self):
+        self.attr2: myint = None  #: docstring