[3.12] gh-86291: linecache: get module name from __spec__ if available (GH-22908) (GH-115731)

This allows getting source code for the __main__ module when a custom
loader is used.
(cherry picked from commit e976baba99a5c243ff3a3b5ef2fd14608a398338)

Co-authored-by: Eugene Toder <eltoder@users.noreply.github.com>