Complete dotted names properly in decorators

Outside decorators, a dotted name is parsed by the grammar as
stmt -> test -> power -> atom trailer -> (NAME) ('.' NAME)
where the first NAME is an 'atom' and the second NAME is a 'trailer'.

Thus, testing if the most recent variable is a 'trailer' and the most
recent node is a '.' is almost always enough for Jedi to properly
complete dotted names.

However, the grammar for decorators is more restricted and doesn't allow
arbitrary atoms and trailers; instead, a dotted name in a decorator is
decorator -> '@' dotted_name -> '@' (NAME '.' NAME),
meaning the most recent variable will be 'dotted_name', not 'trailer'.

Besides in decorators, the 'dotted_name' variable is only used in import
statements which are handled previously in _get_context_completions,
so checking for 'dotted_name' in this arm of the if only covers decorators
and not inadvertently anything else.

jedi/api/completion.py

@@ -164,7 +164,7 @@ class Completion:
                 # No completions for ``with x as foo`` and ``import x as foo``.
                 # Also true for defining names as a class or function.
                 return list(self._get_class_context_completions(is_function=True))
-            elif symbol_names[-1] == 'trailer' and nodes[-1] == '.':
+            elif symbol_names[-1] in ('trailer', 'dotted_name') and nodes[-1] == '.':
                 dot = self._module_node.get_leaf_for_position(self._position)
                 completion_names += self._trailer_completions(dot.get_previous_leaf())
             else: