fix: Fix aliases for direct nested imports

Previously `import a.b.c` was creating an alias
with name `a` pointing to `a.b.c`.
Now it creates an alias with name `a`
pointing to `a`.
We don't care about the nested attributes.

Issue mkdocstrings/python#32: mkdocstrings/python#32

Author: pawamoy

src/griffe/agents/visitor.py

@@ -503,7 +503,7 @@ def visit_import(self, node: ast.Import) -> None:
             node: The node to visit.
         """
         for name in node.names:
-            alias_path = name.name
+            alias_path = name.name.split(".", 1)[0]
             alias_name = name.asname or alias_path.split(".", 1)[0]
             self.current.imports[alias_name] = alias_path
             self.current.set_member(