Issue #13959: Simplify imp.reload() by relying on a module's

Issue #13959: Simplify imp.reload() by relying on a module's
__loader__.

Since import now sets __loader__ on all modules it creates and
imp.reload() already relied on the attribute for modules that import
didn't create, the only potential compatibility issue is if people
were deleting the attribute on modules and expecting imp.reload() to
continue to work.