[ty] Move class_member to member module (#20837)

## Summary

Move the `class_member` function to the `member` module. This allows us
to move the `member` module into the `types` module and to reduce the
visibility of its contents to `pub(super)`. The drawback is that we need
to make `place::place_by_id` public.

## Test Plan

Pure refactoring.

crates/ty_python_semantic/src/lib.rs

@@ -34,7 +34,6 @@ mod db;
 mod dunder_all;
 pub mod lint;
 pub(crate) mod list;
-mod member;
 mod module_name;
 mod module_resolver;
 mod node_key;