Don't drop parent modules if used only in anonsub

Issue 348: Don't drop modules declared in parent namespaces in the
presence usages of modules in child namespaces if the only usage of a
module in a parent namespace is within an anonymous subroutine

ie, if 'Foo' and 'Foo::Bar' exist, and 'Foo::Bar' is used, but 'Foo' is
only used in an anonymous subroutine, do a better job of keeping 'Foo'