also check for the existence of $Package::Name::VERSION in the code before adding another one

(not related to #382 except that the changes here made me realize the errors that are fixed here.)

Does not change behaviour in the success case -- only properly identifies that $Foo::VERSION in the package means that another one should not be added by [PkgVersion] (ironically, the exact form that it adds -- so it is quite possible to use [PkgVersion] twice!).